c++: Overeager use of deleted function before ADL [PR68942]

c++: Overeager use of deleted function before ADL [PR68942]

Here, at template definition time, ordinary name lookup for 'foo(t)'
finds only the deleted function, and so we form a CALL_EXPR thereof.
Later at instantiation time, when initially substituting into this
CALL_EXPR with T=N::A, we end up calling mark_used on this deleted
function (since it's the only function in the overload set), triggering
a bogus "use of deleted function error", before we get to augment the
overload set via ADL.

This patch fixes this issue by using the tf_conv flag to disable
mark_used during the initial substitution into the callee of a
CALL_EXPR when KOENIG_P, since at this point we're still figuring out
which functions are candidates.

gcc/cp/ChangeLog:

	PR c++/68942
	* pt.c (tsubst_copy_and_build) <case CALL_EXPR>: When KOENIG_P,
	set tf_conv during the initial substitution into the function.

gcc/testsuite/ChangeLog:

	PR c++/68942
	* g++.dg/template/koenig12.C: New test.