[clang] [clang] Improve diagnostics for constraints of inline asm (NFC) (PR #96363)

[Evgenii Kudriashov via cfe-commits]

e-kud wrote:

Initially I've implemented the target errors through std::string. Then changed to diag::kind after reading InternalsManual. I'm not sure what is better. The drawback of returning diagnoistics by reference is that we can't customize them, only fixed messages. Maybe this is not a big problem because we don't have constantly changing constraints. 

https://github.com/llvm/llvm-project/pull/96363