[Clang] Produce a warning instead of an error in unevaluated strings before C++26

Emiting an error on unexpected encoding prefix - which was allowed before C++26 -
caused build errors for a few users.
This downgrade the error to a warning on older language modes and C.

Reviewed By: aaron.ballman, hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D156596