[llvm] [clang] [SystemZ] Support i128 as legal type in VRs (PR #74625)

[Ulrich Weigand via cfe-commits]

uweigand wrote:

@JonPsson1 - please have a look at the effects of i128 support in particular on atomics
@redstar - can you check impact on the z/OS ABI?  we may need to handle legal i128 there too, but there doesn't appear to be any in-tree test case for passing i128 on z/OS

Any other comments welcome as well!

https://github.com/llvm/llvm-project/pull/74625