MSVC allows users to pass structures with required alignments greater
than 4 to variadic functions. It does not pass them indirectly to
correctly align them. Instead, it passes them directly with the usual 4
byte stack alignment.
This change implements the same logic in clang on the passing side. The
receiving side (va_arg) never implemented any of this indirect logic, so
it doesn't need to be updated.
This issue pre-existed, but @aaron.ballman noticed it when we started
passing structs containing aligned fields indirectly in D152752.
c8c075e876