d: Fix ICE in expand_intrinsic_vaarg

d: Fix ICE in expand_intrinsic_vaarg

Both intrinsics did not handle the case where the va_list object comes
from a ref parameter.

gcc/d/ChangeLog:

	PR d/96140
	* intrinsics.cc (expand_intrinsic_vaarg): Handle ref parameters as
	arguments to va_arg().
	(expand_intrinsic_vastart): Handle ref parameters as arguments to
	va_start().

gcc/testsuite/ChangeLog:

	PR d/96140
	* gdc.dg/pr96140.d: New test.