[Clang] Backport static_assert messages fixes

* 4d494e76: Handle static_assert messages with an expression started by a literal
* 49e0495f 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