c++/modules: Fix exported using-directive of imported namespace [PR121702]HEADtrunkmaster

Currently we represent exported using-directives as a list of indices
into the namespace array that we stream.  However this list of
namespaces doesn't include any namespaces that we don't expose in this
module's purview, and so we ICE.

This patch reworks the handling to instead use the existing depset
tracking for namespaces directly.  This means that we don't need to
build up a second lookup map when streaming, and we can reuse the logic
in {read,write}_namespace.  We do need to make sure that we create a
depset for namespaces only referenced by a using-directive, though.

I don't expect to be exporting large numbers of using-directives from a
namespace, so for simplicity we stream the names as {parent, target}
pairs.

This also adjusts read handling so that we load the using-directives for
any import (including indirect) if it's in the import list for the
current TU.  Otherwise we run into issues if the using-directive is in
a namespace that is otherwise never referenced in the 'export import'ing
module, because we never walk this namespace and so never know that we
need to emit it.  To do this the patch ensures that we calculate the
import list before read_language is called.

As a drive-by fix, I noticed that with modules 'add_using_namespace'
will add duplicate using-directives because we compare usings against
the target namespace, but we then push a wrapping USING_DECL instead.
This reworks so that the contents of the structure is equivalent between
modules and non-modules code.

	PR c++/121702

gcc/cp/ChangeLog:

	* module.cc (enum module_state_counts): New counter.
	(depset::hash::add_namespace_entities): Seed using-directive
	targets for later streaming.
	(module_state::write_namespaces): Don't handle using-directives
	here.
	(module_state::read_namespaces): Likewise.
	(module_state::write_using_directives): New function.
	(module_state::read_using_directives): New function.
	(module_state::write_counts): Log using-directives.
	(module_state::read_counts): Likewise.
	(module_state::write_begin): Stream using-directives.
	(module_state::read_language): Read using-directives if
	directly importing.
	(module_state::direct_import): Update current TU import list
	before calling read_language.
	* name-lookup.cc (add_using_namespace): Fix lookup of previous
	using-directives.
	* parser.cc (cp_parser_import_declaration): Don't set
	MK_EXPORTING when performing import_module.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/namespace-10_c.C: Add check for log dump.
	* g++.dg/modules/namespace-13_a.C: New test.
	* g++.dg/modules/namespace-13_b.C: New test.
	* g++.dg/modules/namespace-13_c.C: New test.

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

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