c++: *this folding in constexpr call

The code in cxx_eval_call_expression to fold *this was doing the wrong thing
for array decay; we can use cxx_fold_indirect_ref instead.

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_fold_indirect_ref): Add default arg.
	(cxx_eval_call_expression): Call it.
	(cxx_fold_indirect_ref_1): Handle null empty_base.