c++/modules: Stream DECL_CHAIN for decl_specialization_friend_p functions

c++/modules: Stream DECL_CHAIN for decl_specialization_friend_p functions

r16-5298 attached the owning class for a friend template specialisation
on its DECL_CHAIN.  However we don't stream DECL_CHAIN in general to
avoid walking into unrelated entities on the scope chain; this patch
adds a special case for these functions to ensure we don't lose this
information.

Ideally this would occur in trees_{out,in}::core_vals, but we can't
check decl_specialization_friend_p until after DECL_TEMPLATE_INFO has
been streamed, hence the slightly unusual placement.

gcc/cp/ChangeLog:

	* module.cc (trees_out::lang_decl_vals): Stream DECL_CHAIN for
	decl_specialization_friend_p functions.
	(trees_in::lang_decl_vals): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/friend-12_a.C: New test.
	* g++.dg/modules/friend-12_b.C: New test.

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