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

[via cfe-commits]

https://github.com/cor3ntin commented:

Separate default-to-error warning sounds like the right approach given the fallout.

We will need a way to identify that the shadowing declaration is the declaration the template parameter is attached to, so I suspect that change isn't trivial.

I'll revert in a few hours if no one beats me to it.

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