[clang] [llvm] [inlineasm] Add special support for "rm" inline asm constraints (PR #181973)

[Nick Desaulniers via cfe-commits]

nickdesaulniers wrote:

> My recent commit ([48d2a25](https://github.com/llvm/llvm-project/pull/181973/commits/48d2a2513b59d9e04580288a147ea9f6dd6719a1)) demonstrates a major hurdle trying to support the `"g"` constraint. When applied, it causes the CodeGen/RISCV/rvv/pr99782.ll test to fail. I believe this is because the backend isn't properly allowing for the `"i"` constraint in `"imr"`. I think it can be solved, but it will take a _lot_ more time than covering `"rm"`.
> 
> @nickdesaulniers @nikic Are you okay with this patch just supporting `"rm"` and having `"g"` support in a later patch?

I assume "g" isn't valid as _an output_ constraint, only an _input_ constraint?

https://github.com/llvm/llvm-project/pull/181973