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

[Krystian Stasiowski via cfe-commits]

sdkrystian wrote:

Ping @erichkeane (if you are already being pinged automatically please let me know so I'm not double-pinging :) )

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