c++/modules: imported spec befriending class tmpl [PR114889]

c++/modules: imported spec befriending class tmpl [PR114889]

When adding to CLASSTYPE_BEFRIENDING_CLASSES as part of installing an
imported class definition, we need to look through TEMPLATE_DECL like
make_friend_class does.

Otherwise in the below testcase we won't add _Hashtable<int, int> to
CLASSTYPE_BEFRIENDING_CLASSES of _Map_base, which leads to a bogus
access check failure for _M_hash_code.

	PR c++/114889

gcc/cp/ChangeLog:

	* module.cc (trees_in::read_class_def): Look through
	TEMPLATE_DECL when adding to CLASSTYPE_BEFRIENDING_CLASSES.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/friend-8_a.H: New test.
	* g++.dg/modules/friend-8_b.C: New test.

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