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

[via cfe-commits]

Sirraide wrote:

@AaronBallman I just noticed something that I’ve somehow not realised until now even though I’d already written a test case for it: Not only does GCC allow raw string literals in gnuXY mode, but also UTF string literals, e.g. `u"foo"` (https://godbolt.org/z/771s8ne5d). 

Should we follow suit here? And if so, should we add a separate flag for that or rename `fraw-string-literals` (and the LangOption) to `fext-string-literals` or something similar?

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