[clang] [Clang] Allow raw string literals in C as an extension (PR #88265)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

C has Unicode string literals as well: https://godbolt.org/z/chdjYrK9v and so if we're allowing raw string literals, it makes sense to also allow raw unicode string literals IMO. I don't think we need to rename the flag though.

https://github.com/llvm/llvm-project/pull/88265