c++: more overeager use of deleted function before ADL [PR119034]

c++: more overeager use of deleted function before ADL [PR119034]

The PR68942 fix used the tf_conv flag to disable mark_used when
substituting a FUNCTION_DECL callee of an ADL-enabled call.  In this
slightly more elaborate testcase, we end up prematurely calling
mark_used anyway on the FUNCTION_DECL directly from the CALL_EXPR case
of tsubst_expr during partial instantiation, leading to a bogus "use of
deleted function" error.

This patch fixes the general problem in a more robust way by ensuring
the callee of an ADL-enabled call is wrapped in an OVERLOAD, so that
tsubst_expr leaves it alone.

	PR c++/119034
	PR c++/68942

gcc/cp/ChangeLog:

	* pt.cc (tsubst_expr) <case CALL_EXPR>: Revert PR68942 fix.
	* semantics.cc (finish_call_expr): Ensure the callee of an
	ADL-enabled call is wrapped in an OVERLOAD.

gcc/testsuite/ChangeLog:

	* g++.dg/template/koenig13.C: New test.

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