[clang] [llvm] [SystemZ] Support fp16 vector ABI and basic codegen. (PR #171066)

[Ulrich Weigand via cfe-commits]

https://github.com/uweigand approved this pull request.

This version now generally looks good to me, thanks!   A couple of remaining cosmetic issues inline.

The tests show a bit of room for improvement in codegen, but I think this can be handled in follow-on patches where it makes sense.

https://github.com/llvm/llvm-project/pull/171066