[SystemZ] Fix C++ ABI for passing args of structs containing zero width bitfield.

A struct like { float a; int :0; } should per the SystemZ ABI be passed in a
GPR, but to match a bug in GCC it has been passed in an FPR (see 759449c7).

GCC has now corrected the C++ ABI for this case, and this patch for clang
follows suit.

Reviewed By: Ulrich Weigand

Differential Revision: https://reviews.llvm.org/D122388