[clang] [Clang] Warn with -Wpre-c23-compat instead of -Wpre-c++17-compat for u8 character literals in C23 (PR #97210)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> It's a bit weird to have an extension warning given we seem to not support u8 in older language modes at all https://www.godbolt.org/z/1KrqGeT1q
> 
> I think we should support the extension before in addition of having a warning for it @AaronBallman

It's not an extension warning (it's marked `Warning` not `Extension` or `ExtWarn`), and the warning is telling you exactly that -- `u8` in older language modes may not work pre-C23.

https://github.com/llvm/llvm-project/pull/97210