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

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

A vast majority of this is parser related, and I'm not super familiar there, so @cor3ntin is likely a better reviewer here.

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