[Clang] Add warnings for CWG2521

1. Teach -Wuser-defined-literals to warn on using double underscores in
   literal suffix identifiers.
2. Add -Wdeprecated-literal-operator to warn about the use of the first
   grammar production of literal-operator-id, which defaults to off for now.

Differential Revision: https://reviews.llvm.org/D152632