c++/modules: Handle instantiating already tsubsted template friend classes [PR115801]

With modules it may be the case that a template friend class provided
with a qualified name is not found by name lookup at instantiation time,
due to the class not being exported from its module.  This causes issues
in tsubst_friend_class which did not handle this case.

This is caused by the named friend class not actually requiring
tsubsting.  This was already worked around for the "found by name
lookup" case (g++.dg/template/friend5.C), but it looks like there's no
need to do name lookup at all for this particular case to work.

We do need to be careful to continue to do name lookup to handle
templates from an outer current instantiation though; this patch adds a
new testcase for this as well.  This should not impact modules (because
exportingness will only affect namespace lookup).

	PR c++/115801

gcc/cp/ChangeLog:

	* pt.cc (tsubst_friend_class): Return the type immediately when
	no tsubsting or name lookup is required.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/tpl-friend-16_a.C: New test.
	* g++.dg/modules/tpl-friend-16_b.C: New test.
	* g++.dg/template/friend82.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Patrick Palka <ppalka@redhat.com>
Reviewed-by: Jason Merrill <jason@redhat.com>