[AIX] Implement AIX special alignment rule about double/long double

Implement AIX default `power` alignment rule by adding `PreferredAlignment` and
`PreferredNVAlignment` in ASTRecordLayout class.

The patchh aims at returning correct value for `__alignof(x)` and `alignof(x)`
under `power` alignment rules.

Differential Revision: https://reviews.llvm.org/D79719