c++: Fix invalid pointer-to-member in requires [PR67825]

c++: Fix invalid pointer-to-member in requires [PR67825]

A recent change to cmcstl2 led to two tests failing due to this bug: our
valid expression checking in the context of a requires-expression wasn't
catching that an expression of member function type can only appear as the
function operand of a call expression.  Fixed by using convert_to_void to do
the same checking as a discarded-value expression.

This patch also fixes 67825, which already had a testcase, but the testcase
was testing for the wrong behavior.

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

	PR c++/67825
	* constraint.cc (tsubst_valid_expression_requirement): Call
	convert_to_void.