[Clang] Add tests and mark as implemented WG14-N2728

This change expands testing of UTF-8, UTF-16, and UTF-32 character and string
literals as validation that WG14 N2728 (char16_t & char32_t string literals
shall be UTF-16 & UTF-32) has been implemented.

Reviewed By: cor3ntin, aaron.ballman

Differential Revision: https://reviews.llvm.org/D149098