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

[David Blaikie via cfe-commits]

dwblaikie wrote:

> @zmodem template parameter shadowing is an extension with `-fms-compatibility`, if that works

Given the prevalence (wellr, given how quickly folks have tripped over it probably indicates it's fairly prevalent), I think it'll need to be/should be pulled out into its own warning group (which can be a default error) for now at least.

Probably worth reverting while the right path forward is discussed further?

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