c++: mark_used and ADL with template-id [PR100344]

c++: mark_used and ADL with template-id [PR100344]

My r11-295 patch for PR68942 didn't consider that the callee of an
ADL-eligible function call can be a TEMPLATE_ID_EXPR, and we don't want
to disable mark_used when substituting into the template arguments of
this TEMPLATE_ID_EXPR because the arguments are clearly used regardless
of the outcome of ADL.  In the first testcase below, this oversight
causes us to trip over the assert in build_call_a for the call to
find_index<int> because the function no longer had its TREE_USED bit set
from mark_used.

So this patch restricts the original fix to disable mark_used only when
the callee is a FUNCTION_DECL, which seems to be the only case that
matters for PR68942.  For instance, in the second testcase below we
already don't mark_used the deleted function specialization even before
r11-295.

gcc/cp/ChangeLog:

	PR c++/68942
	PR c++/100344
	* pt.c (tsubst_copy_and_build) <case CALL_EXPR>: Set tf_conv
	only when the callee is a FUNCTION_DECL.

gcc/testsuite/ChangeLog:

	PR c++/68942
	PR c++/100344
	* g++.dg/template/call8.C: New test.
	* g++.dg/template/koenig12a.C: New test.