[clang] [Clang][Parse] Diagnose member template declarations with multiple declarators (PR #78243)

[Krystian Stasiowski via cfe-commits]

sdkrystian wrote:

@erichkeane I have refactored `ParseSingleDeclarationAfterTemplate` (now named `ParseDeclarationAfterTemplate`) to call `ParseDeclGroup` and return the resultant `DeclGroup`. 

With respect to moving the point of diagnosis to `FinalizeDeclaratorGroup`: as of now, the diagnostics are still issued from the parser (as it's simpler & results in diagnostics being emitted in lexical order). We can diagnosis this in `FinalizeDeclaratorGroup` if you still deem it necessary. 


https://github.com/llvm/llvm-project/pull/78243