c++: mark_single_function and SFINAE [PR108282]

c++: mark_single_function and SFINAE [PR108282]

We typically ignore mark_used failure when in a non-SFINAE context for
sake of better error recovery.  But in mark_single_function we're
instead ignoring mark_used failure in a SFINAE context, which ends up
causing the second static_assert here to incorrectly fail.

	PR c++/108282

gcc/cp/ChangeLog:

	* decl2.cc (mark_single_function): Ignore mark_used failure
	only in a non-SFINAE context rather than in a SFINAE one.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-requires34.C: New test.