[clang] [clang] Support header shadowing diagnostics in Clang header search (PR #162491)

[Jinjie Huang via cfe-commits]

Jinjie-Huang wrote:

The current implementation directly reuses the search logic from HeaderSearch::LookupFile() and avoids side effects caused by performing additional searches.

Alternatively, we could recalculate the default search paths after locating the corresponding header file and then perform the extra search. This would make the implementation more consistent and better isolate functionalities.

Please let me know if the alternative seems preferable, I’m happy to adjust the implementation.

https://github.com/llvm/llvm-project/pull/162491