codegen tests: add link to LLVM FileCheck docs

Author: RalfJung

src/tests/intro.md

@@ -34,7 +34,8 @@ details.
 - `pretty` – tests targeting the Rust "pretty printer", which
   generates valid Rust code from the AST
 - `debuginfo` – tests that run in gdb or lldb and query the debug info
-- `codegen` – tests that compile and then test the generated LLVM
+- [`codegen`](https://llvm.org/docs/CommandGuide/FileCheck.html) –
+  tests that compile and then test the generated LLVM
   code to make sure that the optimizations we want are taking effect.
 - `assembly` – similar to `codegen` tests, but verifies assembly output
   to make sure LLVM target backend can handle provided code.