Auto merge of rust-lang#51196 - Havvy:remove-keywords, r=petrochenkov

Implement RFC 2421, 'Keyword unreservations (pure, sizeof, alignof, offsetof)

On my local machine I was getting failures in `test/ui-fulldeps`, but I was also getting them again after reverting the change, so I'm hoping that this works.

I removed the test because that's what the other PR that unreserved a keyword did and I feel that it doesn't make sense/keep value to keep a test for something removed years ago.

Author: bors

.gitmodules

@@ -1,58 +0,0 @@
-[submodule "src/llvm"]
-	path = src/llvm
-	url = https://github.com/rust-lang/llvm.git
-	branch = master
-[submodule "src/jemalloc"]
-	path = src/jemalloc
-	url = https://github.com/rust-lang/jemalloc.git
-[submodule "src/rust-installer"]
-	path = src/tools/rust-installer
-	url = https://github.com/rust-lang/rust-installer.git
-[submodule "src/liblibc"]
-	path = src/liblibc
-	url = https://github.com/rust-lang/libc.git
-[submodule "src/doc/nomicon"]
-	path = src/doc/nomicon
-	url = https://github.com/rust-lang-nursery/nomicon.git
-[submodule "src/tools/cargo"]
-	path = src/tools/cargo
-	url = https://github.com/rust-lang/cargo.git
-[submodule "src/doc/reference"]
-	path = src/doc/reference
-	url = https://github.com/rust-lang-nursery/reference.git
-[submodule "src/doc/book"]
-	path = src/doc/book
-	url = https://github.com/rust-lang/book.git
-[submodule "src/tools/rls"]
-	path = src/tools/rls
-	url = https://github.com/rust-lang-nursery/rls.git
-[submodule "src/libcompiler_builtins"]
-	path = src/libcompiler_builtins
-	url = https://github.com/rust-lang-nursery/compiler-builtins
-[submodule "src/tools/clippy"]
-	path = src/tools/clippy
-	url = https://github.com/rust-lang-nursery/rust-clippy.git
-[submodule "src/tools/rustfmt"]
-	path = src/tools/rustfmt
-	url = https://github.com/rust-lang-nursery/rustfmt.git
-[submodule "src/tools/miri"]
-	path = src/tools/miri
-	url = https://github.com/solson/miri.git
-[submodule "src/dlmalloc"]
-	path = src/dlmalloc
-	url = https://github.com/alexcrichton/dlmalloc-rs.git
-[submodule "src/doc/rust-by-example"]
-	path = src/doc/rust-by-example
-	url = https://github.com/rust-lang/rust-by-example
-[submodule "src/llvm-emscripten"]
-	path = src/llvm-emscripten
-	url = https://github.com/rust-lang/llvm
-[submodule "src/stdsimd"]
-	path = src/stdsimd
-	url = https://github.com/rust-lang-nursery/stdsimd
-[submodule "src/tools/lld"]
-	path = src/tools/lld
-	url = https://github.com/rust-lang/lld.git
-[submodule "src/libbacktrace"]
-	path = src/libbacktrace
-	url = https://github.com/rust-lang-nursery/libbacktrace