[Clang] Implement P2513

Implement P2513

This change allows initializing an array of unsigned char,
or char from u8 string literals.
This was done both to support legacy code and for compatibility
with C where char8_t will be typedef to unsigned char.

This is backported to C++20 as per WG21 guidance.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D136449