[clang-tools-extra] [clang] remove ClassScopeFunctionSpecializationDecl (PR #66636)

[Krystian Stasiowski via cfe-commits]

sdkrystian wrote:

Updated the release notes. I didn't go into exhaustive detail, but I can add more if anyone here deems it necessary @erichkeane @AaronBallman 

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