[clang] [clang][lex] Deduplicate `-Wheader-shadowing` diags (PR #216748)

[Jinjie Huang via cfe-commits]

Jinjie-Huang wrote:

Thanks for the patch. By the way, just out of curiosity, did you run into this issue because you are actively using this warning in projects?

https://github.com/llvm/llvm-project/pull/216748