Run clang-tidy on headers (#3572)

This patches bazel_clang_tidy handling of headers. I found an equivalent
change at https://github.com/erenon/bazel_clang_tidy/pull/13, but that
was [already
rejected](https://github.com/erenon/bazel_clang_tidy/pull/13#issuecomment-1047007424).
Per the criticism, this will result in redundant processing of headers.

The project instead uses `HeaderFilterRegex: ".*"`, but that results in
two problems:

1. When running with `-k`, errors are repeated when a header is included
more than once, which is common.
2. clang-tidy including errors from headers that are included from other
modules (e.g., abseil-cpp); filtering correctly is difficult.

Given the trade-offs and options (including forking), I thought patching
was preferable so long as it remains narrow.

common/init_llvm.cpp

@@ -34,6 +34,6 @@ InitLLVM::InitLLVM(int& argc, char**& argv)
   }
 }
 
-auto (*InitLLVM::InitializeTargets)() -> void = nullptr;
+InitLLVM::InitializeTargetsFn* InitLLVM::InitializeTargets = nullptr;
 
 }  // namespace Carbon