c++: avoid ICE with __builtin_memset (PR90997).

warn_for_memset calls fold_for_warn, which calls fold_non_dependent_expr, so
also calling instantiate_non_dependent_expr here is undesirable.

gcc/cp/ChangeLog
2020-03-04  Jason Merrill  <jason@redhat.com>

	PR c++/90997
	* semantics.c (finish_call_expr): Don't call
	instantiate_non_dependent_expr before warn_for_memset.