c++/modules: Fix revealing with using-decls [PR114867]HEADtrunkmaster

This patch fixes a couple issues with the current handling of revealing
declarations with using-decls.

Firstly, doing 'remove_node' when handling function overload sets is not
safe, because it not only mutates the OVERLOAD we're walking over but
potentially any other references to this OVERLOAD that are cached from
phase-1 template lookup.  This causes the attached using-17 testcase to
fail because the overload set in 'X::test()' no longer contains the
'ns::f(T)' template once instantiated at the end of the file.

This patch works around this by simply not removing the old declaration.
This does make the overload list potentially longer than it otherwise
would have been, but only when re-exporting the same set of functions in
a using-decl.  Additionally, because 'ovl_insert' always prepends these
newly inserted overloads, repeated exported using-decls won't continue
to add declarations, as the first exported using-decl will be found
before the original (unexported) declaration.

Another, related, issue is that using-decls of GMF entities currently
doesn't mark them as reachable unless they are also exported, and thus
they may not be available in e.g. module implementation units.  We solve
this with a new flag on OVERLOADs set when they are declared within the
module purview.  This starts to run into the more general issue of
handling using-decls of non-functions (see e.g. PR114863) but by just
marking such GMF entities as purview we can work around this for now.

This also allows us to get rid of the special-casing of exported
using-decls in 'add_binding_entity', which was incorrect anyway: a
non-exported using-decl still needs to be emitted anyway if it lives in
the module purview, even if referring to a non-purview item.

	PR c++/114867

gcc/cp/ChangeLog:

	* cp-tree.h (OVL_PURVIEW_P): New.
	(ovl_iterator::purview_p): New.
	* module.cc (depset::hash::add_binding_entity): Only ignore
	entities not within module purview. Set OVL_PURVIEW_P on new
	OVERLOADs for emitted declarations.
	(module_state::read_cluster): Imported using-decls are always
	in purview, mark as OVL_PURVIEW_P.
	* name-lookup.h (enum WMB_Flags): New WMB_Purview flag.
	* name-lookup.cc (walk_module_binding): Set WMB_Purview as
	needed.
	(do_nonmember_using_decl): Don't remove from existing OVERLOADs.
	Also reveal non-exported decls. Also reveal 'extern "C"' decls.
	Add workaround to reveal non-function decls.
	* tree.cc (ovl_insert): Adjust to also set OVL_PURVIEW_P when
	needed.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/using-17_a.C: New test.
	* g++.dg/modules/using-17_b.C: New test.
	* g++.dg/modules/using-18_a.C: New test.
	* g++.dg/modules/using-18_b.C: New test.

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

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