[clang] [Clang][Sema] Diagnose function/variable templates that shadow their own template parameters (PR #78274)

[Krystian Stasiowski via cfe-commits]

sdkrystian wrote:

@erichkeane I don't think I can add reviewers [without write access to the repo](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/requesting-a-pull-request-review) unfortunately

https://github.com/llvm/llvm-project/pull/78274