[clang] [llvm] [Clang][LoongArch] Add inline asm support for the `q` constraint (PR #141037)

[via cfe-commits]

heiher wrote:

> I have a C test case:
> 
> ```c
> unsigned long
> test (unsigned long x, unsigned long y)
> {
>   register unsigned long ra asm ("ra");
>   asm("" : "+r"(ra));
> 
>   unsigned long t = ra;
>   asm ("csrxchg %0, %1, 0" : "+r" (x) : "r" (t));
> 
>   return x;
> }
> ```
> 
> clang-20.1.4 errors out:
> 
> ```
> t.c:8:8: error: must not be $r0 or $r1
>     8 |   asm ("csrxchg %0, %1, 0" : "+r" (x) : "r" (t));
>       |        ^
> <inline asm>:1:16: note: instantiated into assembly here
>     1 |         csrxchg $a0, $ra, 0
>       |                       ^
> 1 error generated.
> ```
> 
> And I suppose it should work after this PR and changing the second `"r"` to `"q"`.

Good idea!

https://github.com/llvm/llvm-project/pull/141037