[llvm] [CodeGen] Enhance inline asm constraint diagnostics (PR #101354)

[Evgenii Kudriashov via llvm-commits]

e-kud wrote:

This is a continuation of https://github.com/llvm/llvm-project/pull/96363. Here I want backends to provide additional information why a register can't be allocated.

My points of concerns:
* I don't like using `std::string` for error messages.
* I've tried to return `Expected<std::pair<unsigned, const TargetRegisterClass *>>` from `getRegForInlineAsmConstraint`. But this is much more intrusive change than simply adding the reference argument. Because we need to update all return statements in all backends.
* In case of `X86`, `TargetLowering::getRegForInlineAsmConstraint` may override the error message obtained from switches before. But it's not a problem to preserve a message if any, and ignore the one from generic version.

Hi @arsenm @RKSimon @efriedma-quic @phoebewang what do you think? Are there better ways to make backends more friendly in terms of diagnostics.

This is a draft. When or if we agree with the approach, I'll cover all failures from `getRegForInlineAsmConstraint` for `X86`. For now I've covered only cases from tests.

https://github.com/llvm/llvm-project/pull/101354