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

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I guess my first question is 'why'?  My second is 'if this is unnecessary, why was it here in the first place?`.  I'd like some level of historical analysis here/perhaps the knowledge of @zygoloid.

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