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

[Fangrui Song via cfe-commits]

MaskRay wrote:

I also haven't taken the time to fully comprehend the source code changes. Both libcxx and glibc ship `stdio.h`. How do you handle `#include <stdio.h>`? Probing every search path also has a potential syscall performance issue.

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