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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > I don't think we should allow it for C++11 and later modes
> 
> To clarify, should we allow enabling them in e.g. `c++03` mode if `-fraw-string-literals` is passed? I don’t see why not, but I’m not entirely sure whether you’re saying we should not support that flag in C++ (neither the positive nor the negative variant) at all or just not in C++11 and later.

I think we should allow users to enable them in C++03 modes if `-fraw-string-literals` is passed. I think it's fine to have `-fno-raw-string-literals` that allows users to disable them in C++03 mode (in case an earlier command line option opted into them and the user wants to disable the feature for some reason), but I don't know if that's worth the effort to support because I don't think we should allow `-fno-raw-string-literals` in C++11 mode.

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