[clang] Reapply "[Clang][Sema] Diagnose function/variable templates that shadow their own template parameters (#78274)" (PR #79683)

[Krystian Stasiowski via cfe-commits]

sdkrystian wrote:

Need to run clang-format on this and mention the flag in the release notes, but should be otherwise GTG

https://github.com/llvm/llvm-project/pull/79683