c++: Propagate hidden flag on decls from partitions

c++: Propagate hidden flag on decls from partitions

While working on some other fixes I noticed that the partition handling
code used the wrong flag to propagate OVL_HIDDEN_P on exported bindings
from partitions. This patch fixes that, and renames the flag to be
clearer.

gcc/cp/ChangeLog:

	* name-lookup.cc (walk_module_binding): Use the
	partition-specific hidden flag instead of the top-level
	decl_hidden.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/using-16_a.C: New test.
	* g++.dg/modules/using-16_b.C: New test.
	* g++.dg/modules/using-16_c.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>