c++/modules: Also check conflicting internal-linkage entities

c++/modules: Also check conflicting internal-linkage entities

While investigating another issue I noticed that the condition in
check_module_override seems incorrect: the wording in [basic.link] p11
has no exceptions for internal-linkage entities.

gcc/cp/ChangeLog:

	* name-lookup.cc (check_module_override): Remove check for
	TREE_PUBLIC when checking mergeable entities.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/namespace-1_c.C: Adjust to expect errors.
	* g++.dg/modules/namespace-2_b.C: Likewise.
	* g++.dg/modules/namespace-3_a.C: Removed.
	* g++.dg/modules/namespace-3_b.C: Removed.

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