[FE] Use preferred alignment instead of ABI alignment for complete object when applicable

On some targets, preferred alignment is larger than ABI alignment in some cases. For example,
on AIX we have special power alignment rules which would cause that. Previously, to support
those cases, we added a “PreferredAlignment” field in the `RecordLayout` to store the AIX
special alignment values in “PreferredAlignment” as the community suggested.

However, that patch alone is not enough. There are places in the Clang where `PreferredAlignment`
should have been used instead of ABI-specified alignment. This patch is aimed at fixing those
spots.

Differential Revision: https://reviews.llvm.org/D86790