[clang] Fix emitVoidPtrVAArg for non-zero default alloca address space

Indirect arguments are passed on the stack and so va_arg should use the
default alloca address space, not hard-code 0, for pointers to those.
The only in-tree target with a non-zero default alloca address space is
AMDGPU, but that does not support variadic arguments, so we cannot test
this upstream. However, downstream in CHERI LLVM (and Morello LLVM, a
further fork of that) we have targets that do both and so require this
change.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D132247