c++/modules: Mark implicit using-directive for imported unnamed namespace as imported...

c++/modules: Mark implicit using-directive for imported unnamed namespace as imported [PR123393]

The assertion failure in the PR is due to the implicit using-directive
for the anonymous namespace not being considered imported, because
make_namespace_finish calls add_using_namespace without propagating
'from_import'.

	PR c++/123393

gcc/cp/ChangeLog:

	* name-lookup.cc (make_namespace_finish): Pass from_import to
	add_using_namespace.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/namespace-18_a.C: New test.
	* g++.dg/modules/namespace-18_b.C: New test.

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