c++: compound-requirement partial substitution [PR113966]

When partially substituting a requires-expr, we don't want to perform
any additional checks beyond the substitution itself so as to minimize
checking requirements out of order.  So don't check the return-type-req
of a compound-requirement during partial substitution.  And don't check
the noexcept condition either since we can't do that on templated trees.

	PR c++/113966

gcc/cp/ChangeLog:

	* constraint.cc (tsubst_compound_requirement): Don't check
	the noexcept condition or the return-type-requirement when
	partially substituting.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-friend17.C: New test.

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