[clang] [llvm] Add support for flag output operand "=@cc" for SystemZ. (PR #125970)

[Ulrich Weigand via cfe-commits]

https://github.com/uweigand commented:

This now looks good, I cannot see any functional issues any more.  Two last remaining cosmetic issues pointed out inline.  Otherwise, we only need a more complete commit message and then this looks good to go.

https://github.com/llvm/llvm-project/pull/125970