[llvm] Add diagnostic help for inline asm operand constraint 'H' (PR #86910)

[via llvm-commits]

mahesh-attarde wrote:

> I think it would be a better API to migrate the print from return bool to Error instead of adding a string out argument

This is tricky. AsmPrinter::PrintAsmOperand is overridden in each target. 'H' can be slightly different  for each target. So when we return to caller of PrintAsmOperand ( generalization) we may need separate special message as per Target. To support it, i added std::string parameter.  Is that okay? 

https://github.com/llvm/llvm-project/pull/86910