[clang] [AArch64] Fix handling of HFAs passed to Windows variadic functions

The documentation says that for variadic functions, all composites
are treated similarly, no special handling of HFAs/HVAs, not even
for the fixed arguments of a variadic function.

Differential Revision: https://reviews.llvm.org/D100467

GitOrigin-RevId: 8e0f2e89ff951c74875ed751e2215cc263b33328
2 files changed