c++: ICE during requires-expr partial subst [PR118060]

c++: ICE during requires-expr partial subst [PR118060]

Here during partial substitution of the requires-expression (as part of
CTAD constraint rewriting) we segfault from the INDIRECT_REF case of
convert_to_void due *f(u) being type-dependent.  We should just defer
checking convert_to_void until satisfaction.

	PR c++/118060

gcc/cp/ChangeLog:

	* constraint.cc (tsubst_valid_expression_requirement): Don't
	check convert_to_void during partial substitution.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-requires40.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>