[clang] [clang] remove ClassScopeFunctionSpecializationDecl (PR #66636)

[Krystian Stasiowski via cfe-commits]

sdkrystian wrote:

@AaronBallman 
> Can you be sure to file an issue or add a test case + FIXME comment so we don't lose track of the needed follow-up work, or are you planning to do that work immediately after this lands?

I will address that once this gets merged

https://github.com/llvm/llvm-project/pull/66636