Update clang tidy Contributing guide (#106672)

Update the documentation to direct new users to the Github instead of
the discontinued Phabricator archive. Also details more ways and
information regarding clang-query usage. Partially resolves/disclaims
#106656 and #106663 as per discussion in
https://discourse.llvm.org/t/inconsistency-between-hasdescendant-in-clang-query-and-clang-libtooling-matchers/80799/.

Also updates the out-of-tree guide.

For context, I recently went through the Contributing guide while
writing #102299, and many of
these updates were from my experience trying to follow the guide. e.g. I
was trying to link the shared library of an out-of-tree check as SHARED
in CMake and encountered duplicate symbols like
_ZTIN5clang4tidy14ClangTidyCheckE. It wasn't until I saw
84f137a
that I found out I had to use MODULE. I also encountered the clang-query
difference which was a surprise as the documentation said the two
matchers were "virtually identical". Also, the -header-filter thing
tripped me out until I found
#25590 and
#91400. Usually, when people
say restrict and filter, they mean filter out (since -header-filter
instead includes/filters in said headers).

Author: MichelleCDjunaidi

clang-tools-extra/docs/clang-tidy/Contributing.rst

@@ -127,14 +127,15 @@ Writing a clang-tidy Check
 
 So you have an idea of a useful check for :program:`clang-tidy`.
 
-First, if you're not familiar with LLVM development, read through the `Getting
-Started with LLVM`_ document for instructions on setting up your workflow and
+First, if you're not familiar with LLVM development, read through the `Getting Started 
+with the LLVM System`_ document for instructions on setting up your workflow and
 the `LLVM Coding Standards`_ document to familiarize yourself with the coding
-style used in the project. For code reviews we mostly use `LLVM Phabricator`_.
+style used in the project. For code reviews we currently use `LLVM Github`_,
+though historically we used Phabricator.
 
-.. _Getting Started with LLVM: https://llvm.org/docs/GettingStarted.html
+.. _Getting Started with the LLVM System: https://llvm.org/docs/GettingStarted.html
 .. _LLVM Coding Standards: https://llvm.org/docs/CodingStandards.html
-.. _LLVM Phabricator: https://llvm.org/docs/Phabricator.html
+.. _LLVM Github: https://github.com/llvm/llvm-project
 
 Next, you need to decide which module the check belongs to. Modules
 are located in subdirectories of `clang-tidy/
@@ -336,13 +337,24 @@ a starting point for your test cases.  A rough outline of the process looks like
 The quickest way to prototype your matcher is to use :program:`clang-query` to
 interactively build up your matcher.  For complicated matchers, build up a matching
 expression incrementally and use :program:`clang-query`'s ``let`` command to save named
-matching expressions to simplify your matcher.  Just like breaking up a huge function
-into smaller chunks with intention-revealing names can help you understand a complex
-algorithm, breaking up a matcher into smaller matchers with intention-revealing names
-can help you understand a complicated matcher.  Once you have a working matcher, the
-C++ API will be virtually identical to your interactively constructed matcher.  You can
-use local variables to preserve your intention-revealing names that you applied to
-nested matchers.
+matching expressions to simplify your matcher.
+
+.. code-block:: console
+  clang-query> let c1 cxxRecordDecl()
+  clang-query> match c1
+
+Alternatively, pressing the tab key after a previous matcher's open parentheses would also 
+show which matchers can be chained with the previous matcher, though some matchers that work 
+may not be listed.
+
+Just like breaking up a huge function into smaller chunks with intention-revealing names 
+can help you understand a complex algorithm, breaking up a matcher into smaller matchers 
+with intention-revealing names can help you understand a complicated matcher.  
+
+Once you have a working clang-query matcher, the C++ API matchers will be the same or similar 
+to your interactively constructed matcher (there can be cases where they differ slightly). 
+You can use local variables to preserve your intention-revealing names that you applied 
+to nested matchers.
 
 Creating private matchers
 ^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -646,10 +658,13 @@ directory.  The path to this directory is available in a lit test with the varia
 Out-of-tree check plugins
 -------------------------
 
+
 Developing an out-of-tree check as a plugin largely follows the steps
-outlined above. The plugin is a shared library whose code lives outside
+outlined above, including creating a new module and doing the hacks to 
+register the module. The plugin is a shared library whose code lives outside
 the clang-tidy build system. Build and link this shared library against
-LLVM as done for other kinds of Clang plugins.
+LLVM as done for other kinds of Clang plugins. If using CMake, use the keyword
+``MODULE`` while invoking ``add_library`` or ``llvm_add_library``.
 
 The plugin can be loaded by passing `-load` to `clang-tidy` in addition to the
 names of the checks to enable.
@@ -664,6 +679,19 @@ compiled against the version of clang-tidy that will be loading the plugin.
 The plugins can use threads, TLS, or any other facilities available to in-tree
 code which is accessible from the external headers.
 
+Note that testing out-of-tree checks might involve getting ``llvm-lit`` from an LLVM 
+installation compiled from source. See `Getting Started with the LLVM System`_ for ways 
+to do so.
+
+Alternatively, get `lit`_ following the `test-suite guide`_ and get the `FileCheck`_ binary, 
+and write a version of `check_clang_tidy.py`_ to suit your needs.
+
+.. _Getting Started with the LLVM System: https://llvm.org/docs/GettingStarted.html
+.. _test-suite guide: https://llvm.org/docs/TestSuiteGuide.html
+.. _lit: https://llvm.org/docs/CommandGuide/lit.html
+.. _FileCheck: https://llvm.org/docs/CommandGuide/FileCheck.html
+.. _check_clang_tidy.py: https://github.com/llvm/llvm-project/blob/main/clang-tools-extra/test/clang-tidy/check_clang_tidy.py
+
 Running clang-tidy on LLVM
 --------------------------
 
@@ -688,10 +716,10 @@ warnings and errors. The script provides multiple configuration flags.
 
 * To restrict the files examined you can provide one or more regex arguments
   that the file names are matched against.
-  ``run-clang-tidy.py clang-tidy/.*Check\.cpp`` will only analyze clang-tidy
+  ``run-clang-tidy.py clang-tidy/.*Check\.cpp`` will only analyze `clang-tidy`
   checks. It may also be necessary to restrict the header files that warnings
-  are displayed from using the ``-header-filter`` flag. It has the same behavior
-  as the corresponding :program:`clang-tidy` flag.
+  are displayed from by using the ``-header-filter`` and ``-exclude-header-filter`` flags. 
+  They have the same behavior as the corresponding :program:`clang-tidy` flags.
 
 * To apply suggested fixes ``-fix`` can be passed as an argument. This gathers
   all changes in a temporary directory and applies them. Passing ``-format``
@@ -758,4 +786,4 @@ There is only one argument that controls profile storage:
 
   * If you run :program:`clang-tidy` from within ``/foo`` directory, and specify
     ``-store-check-profile=.``, then the profile will still be saved to
-    ``/foo/<ISO8601-like timestamp>-example.cpp.json``
+    ``/foo/<ISO8601-like timestamp>-example.cpp.json``