[ARM, AArch64]: Use unadjusted alignment when passing composites as arguments

The "Procedure Call Procedure Call Standard for the ARM® Architecture"
(https://static.docs.arm.com/ihi0042/f/IHI0042F_aapcs.pdf), specifies that
composite types are passed according to their "natural alignment", i.e. the
alignment before alignment adjustment on the entire composite is applied.

The same applies for AArch64 ABI.

Clang, however, used the adjusted alignment.

GCC already implements the ABI correctly. With this patch Clang becomes
compatible with GCC and passes such arguments in accordance with AAPCS.

Differential Revision: https://reviews.llvm.org/D46013

llvm-svn: 338279