[Clang] Implement static operator[]

After accepted in Kona, update the code to accept static operator[] as well.

No big code changes: accept this operator as static in SemaDeclCXX, update AST call generation in SemaOverload and update feature macros + tests accordingly.

Reviewed By: cor3ntin, erichkeane, #clang-language-wg

Differential Revision: https://reviews.llvm.org/D138387