[Clang][Sema] Fix display of characters on static assertion failure

This patch fixes the display of characters appearing in LHS or RHS of == expression in notes to static assertion failure.
This applies C-style escape if the printed character is a special character. This also adds a numerical value displayed next to the character representation.
This also tries to print multi-byte characters if the user-provided expression is multi-byte char type.

Reviewed By: cor3ntin
Differential Revision: https://reviews.llvm.org/D155610