[clang] [llvm] [clang][AVR] Improve compatibility of inline assembly with avr-gcc (PR #136534)

[Ben Shi via cfe-commits]

benshi001 wrote:

> > > gcc doesn't seem to allow it? https://godbolt.org/z/4zh8TTPac
> > 
> > 
> > avr-gcc allow value 64 for constraint 'I' in very special case, such as #51513.
> 
> I guess the test needs to be updated to be representative?
> 
> > And my solution is loosen that check in the frontend, but let the AVR backend to deny the illegal constraint value 64.
> 
> This deserves a comment in code so that other people don't get confused about this and don't try to change it back to 63.

Thanks.

1. Comment lines are supplemented.
2. A reject test in the backend is added.

https://github.com/llvm/llvm-project/pull/136534