[ARM][AArch64] Fix incorrect handling of alignment in va_arg code generation
Overaligned and underaligned types (i.e. types where the alignment has been
increased or decreased using the aligned and packed attributes) weren't being
correctly handled in all cases, as the unadjusted alignment should be used.
This patch also adjusts getTypeUnadjustedAlign to correctly handle typedefs of
non-aggregate types, which it appears it never had to handle before.
Differential Revision: https://reviews.llvm.org/D62152