[PowerPC][AIX} FIx AIX bootstrap build.

A recent patch:
https://reviews.llvm.org/rGe0921655b1ff8d4ba7c14be59252fe05b705920e
changed clangs AIX bitfield handling to use 4-byte bitfield containers,
matching XLs behavior. This change triggers static assert failures when
bootstrapping. Change the macro we check to enable bitfield packing on
AIX to `__clang__` which is defined by both xlclang and clang.

Differential Revision: https://reviews.llvm.org/D103474