[clang] [AArch64] Fix Windows va_arg handling for larger structs

Aggregate types over 16 bytes are passed by reference.

Contrary to the x86_64 ABI, smaller structs with an odd (non power
of two) are padded and passed in registers.

Differential Revision: https://reviews.llvm.org/D100374

GitOrigin-RevId: 3637c5c8ec3d4dc0b87eb4e3ee9c9ae8816cade2
2 files changed