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

[Krystian Stasiowski via cfe-commits]

sdkrystian wrote:

@cor3ntin @AaronBallman I can work on reapplying this tomorrow. Do we want a flag that allows _all_ template parameter shadowing as a warning, or only in this particular case? 

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