c++: CWG2369 workaround and ... [PR120185]

c++: CWG2369 workaround and ... [PR120185]

My r16-479 adjustment to the PR99599 workaround broke on a class with a
varargs constructor.

It also occurred to me that we don't need to do non-dep conversion checking
in two phases when concepts aren't supported.

	PR c++/99599
	PR c++/120185

gcc/cp/ChangeLog:

	* class.cc (type_has_converting_constructor): Handle null parm.
	* pt.cc (fn_type_unification): Skip early non-dep checking if
	no concepts.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-nondep6.C: New test.