[AIX] "aligned" attribute should not decrease type alignment returned by __alignof__

`__alignof__(x)` always returns `ABIAlign` if the "x" is marked `__attribute__((aligned()))`. However, the "aligned" attribute should only increase the alignment of a struct, or struct member, unless it's used together with the "packed" attribute, or used as a part of a typedef, in which case, the "aligned" attribute can both increase and decrease alignment.

Reviewed By: sfertile

Differential Revision: https://reviews.llvm.org/D107598