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

[Qiu Chaofan via cfe-commits]

https://github.com/ecnelises approved this pull request.

I tested this patch with LLVM trunk bootstrap on Debian 12 and macOS 26, no false positive found.

This looks good to me now, thanks. Please hold on a few days in case other reviewers have more comments.

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