c++/modules: Fix stream-in of member using-decls [PR120414]

When streaming in a reference to a data member, we have an oversight
where we did not consider USING_DECLs, despite otherwise handling them
here the same as fields.  This patch corrects that mistake.

	PR c++/120414

gcc/cp/ChangeLog:

	* module.cc (trees_in::tree_node): Allow reading a USING_DECL
	when streaming tt_data_member.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/using-31_a.C: New test.
	* g++.dg/modules/using-31_b.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>
(cherry picked from commit 43dddeef7a870ce4db7407f73660504b67a0a919)