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

[via cfe-commits]

Sirraide wrote:

Alright, I think this has the behaviour that we want now:
- raw string literals are enabled in C++11 and later, as well as in C in gnuXY mode;
- raw string literals can be explicitly enabled or disabled in C (and in C++ standards before C++11) using `-f[no-]raw-string-literals`;
- in C++11 and later `-f[no-]raw-string-literals` is ignored and a warning is issued.

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