[clang-tools-extra] [clangd] Index reserved symbols from `*intrin.h` system headers (PR #119735)

[Nathan Ridge via cfe-commits]

https://github.com/HighCommander4 requested changes to this pull request.

Thanks for the patch. I thought about whether it makes sense to have an additional check for the `*intrin.h` header being inside the resource directory specifically, but I don't see any headers with that name ending in any other system include directories, so I don't think we need to bother.

Would you mind adding a test case to `SymbolCollectorTests.cpp` as well? [Here](https://searchfox.org/llvm/rev/0cbdad4bd2396b740742d9ae94cba7d7b8a32cb5/clang-tools-extra/clangd/unittests/SymbolCollectorTests.cpp#1376)'s an example of a test customizing a header file name, and [here](https://searchfox.org/llvm/rev/0cbdad4bd2396b740742d9ae94cba7d7b8a32cb5/clang-tools-extra/clangd/unittests/SymbolCollectorTests.cpp#2110) of exercising the "is system header" codepath.

https://github.com/llvm/llvm-project/pull/119735