c++/modules: Handle naming external TU-local entities in ADLHEADtrunkmaster

This finishes the reworking of ADL handling for modules for PR117658.
[basic.link] p18 says that we should diagnose any references to a
TU-local entity from a different TU; with our fixed handling of ADL this
is now possible to occur.

To do this we need to generate fake bindings for these decls on
stream-out so that importers know that we have a visible name in this
namespace.  We don't actually need (or want) to provide full DECLs
though, as that could potentially involve streaming other TU-local
entities, so we just build a TU_LOCAL_ENTITY for the decl on receipt.
The patch also uses 'decl_as_string' to give a more descriptive name for
these decls when erroring.

We also need somewhere to store these decls.  We only actually need the
decls for diagnostics; for correctness we only need to know whether any
such decls existed, so to not mess with the existing packing of bindings
or usages of the OVERLOADs this patch adds a new map to the binding
vector that can be looked up when diagnostics need to be generated.

Finally, as specified this diagnostic is a pretty broad hammer, as any
internal-linkage purview function will stop ADL in exported templates
from working with that name.  So this patch just makes it a pedwarn and
provides an option to disable if needed.

	PR c++/117658

gcc/c-family/ChangeLog:

	* c.opt: New flag '-Wexternal-tu-local'.
	* c.opt.urls: Regenerate.

gcc/cp/ChangeLog:

	* cp-tree.h (TU_LOCAL_ENTITY_NAME): Clarify meaning.
	* module.cc (depset::entity_kind): New enumerator, assert that
	we have enough bits reserved.
	(depset::disc_bits): Assert the discriminator has enough bits.
	(depset::entity_kind_name): Add 'tu-local' case, assert we
	have an entry for all relevant entry_kinds.
	(name_for_tu_local_decl): New function.
	(trees_out::tree_node): Use it.
	(depset::hash::make_dependency): Validate EK_TU_LOCAL.
	(depset::hash::add_binding_entity): Generate bindings for
	internal purview functions.
	(enum ct_bind_flags): New enum for TU-local decls.
	(depset::hash::find_dependencies): Handle EK_TU_LOCAL entities.
	(binding_cmp): Likewise.
	(sort_cluster): Likewise.
	(module_state::write_cluster): Likewise.
	(module_state::read_cluster): Likewise.
	* name-lookup.cc (append_imported_binding_slot): Propagate
	internal decl list when growing binding vector.
	(name_lookup::adl_namespace_fns): Diagnose if naming a TU-local
	entity from a different TU.
	(set_module_binding): Include any internal decls in binding.
	* name-lookup.h (struct module_tree_map_traits): New type.
	(struct tree_binding_vec): Add member 'internal_decls'.
	(BINDING_VECTOR_INTERNAL_DECLS): New getter.
	(MODULE_BINDING_INTERNAL_DECLS_P): New flag.
	(set_module_binding): Add parameter.

gcc/ChangeLog:

	* doc/invoke.texi: Document '-Wno-external-tu-local'.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/adl-6_c.C: Adjust diagnostics.
	* g++.dg/modules/internal-14_c.C: Likewise.
	* g++.dg/modules/internal-15_a.C: New test.
	* g++.dg/modules/internal-15_b.C: New test.

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

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