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

[Evgenii Kudriashov via cfe-commits]

e-kud wrote:

So, I think this PR still makes sense but without target changes, right?

I've taken a look at the backend and constraints are checked in `getRegForInlineAsmConstraint`. We either need to return an error message or pass `Context` into it. The former is preferrable because a call of `getRegForInlineAsmConstraint` doesn't always mean an error is requried. However, in both cases we need to touch all the targets...

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