c++: auto function as function argument [PR105779]

This testcase demonstrates that the issue in PR105623 is not limited to
templates, so we should do the marking in a less template-specific place.

	PR c++/105779

gcc/cp/ChangeLog:

	* call.cc (resolve_args): Call mark_single_function here.
	* pt.cc (unify_one_argument): Not here.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/auto-fn63.C: New test.