c++/modules: Clean up importer_interface

c++/modules: Clean up importer_interface

This patch removes some no longer needed special casing in linkage
determination, and makes the distinction between "always_emit" and
"internal" for better future-proofing.

gcc/cp/ChangeLog:

	* module.cc (importer_interface): Adjust flags.
	(get_importer_interface): Rename flags.
	(trees_out::core_bools): Clean up special casing.
	(trees_out::write_function_def): Rename flag.

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