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

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alexfh wrote:

We've got a huge amount of fallout from this change too. The cleanup would require a lot of work. This compiler error (though useful) definitely needs to be a diagnostic with a separate disable flag with at least one release of grace period. For now, I suggest to revert the change until this is implemented.

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