Expunge namespace-scope IDENTIFIER_TYPE_VALUE & global_type_name [PR 99039]

Expunge namespace-scope IDENTIFIER_TYPE_VALUE & global_type_name [PR 99039]

IDENTIFIER_TYPE_VALUE and friends is a remnant of G++'s C origins.  It
holds elaborated types on identifier-nodes.  While this is fine for C
and for local and class-scopes in C++, it fails badly for namespaces.
In that case a marker 'global_type_node' was used, which essentially
signified 'this is a namespace-scope type *somewhere*', and you'd have
to do a regular name_lookup to find it.  As the parser and
substitution machinery has avanced over the last 25 years or so,
there's not much outside of actual name-lookup that uses that.
Amusingly the IDENTIFIER_HAS_TYPE_VALUE predicate will do an actual
name-lookup and then users would repeat that lookup to find the
now-known to be there type.

Rather late I realized that this interferes with the lazy loading of
module entities, because we were setting IDENTIFIER_TYPE_VALUE to
global_type_node.  But we could be inside some local scope where that
identifier is bound to some local type.  Not good!

Rather than add more cruft to look at an identifier's shadow stack and
alter that as necessary, this takes the approach of removing the
existing cruft.

We nuke the few places outside of name lookup that use
IDENTIFIER_TYPE_VALUE.  Replacing them with either proper name
lookups, alternative sequences, or in some cases asserting that they
(no longer) happen.  Class template instantiation was calling pushtag
after setting IDENTIFIER_TYPE_VALUE in order to stop pushtag creating
an implicit typedef and pushing it, but to get the bookkeeping it
needed.  Let's just do the bookkeeping directly.

Then we can stop having a 'bound at namespace-scope' marker at all,
which means lazy loading won't screw up local shadow stacks.  Also, it
simplifies set_identifier_type_value_with_scope, as it never needs to
inspect the scope stack.  When developing this patch, I discovered a
number of places we'd put an actual namespace-scope type on the
type_value slot, rather than global_type_node.  You might notice this
is killing at least two 'why are we doing this?' comments.

While this doesn't fix the two PRs mentioned, it is a necessary step.

	PR c++/99039
	PR c++/99040
	gcc/cp/
	* cp-tree.h (CPTI_GLOBAL_TYPE): Delete.
	(global_type_node): Delete.
	(IDENTIFIER_TYPE_VALUE): Delete.
	(IDENTIFIER_HAS_TYPE_VALUE): Delete.
	(get_type_value): Delete.
	* name-lookup.h (identifier_type_value): Delete.
	* name-lookup.c (check_module_override): Don't
	SET_IDENTIFIER_TYPE_VALUE here.
	(do_pushdecl): Nor here.
	(identifier_type_value_1, identifier_type_value): Delete.
	(set_identifier_type_value_with_scope): Only
	SET_IDENTIFIER_TYPE_VALUE for local and class scopes.
	(pushdecl_nanmespace_level): Remove shadow stack nadgering.
	(do_pushtag): Use REAL_IDENTIFIER_TYPE_VALUE.
	* call.c (check_dtor_name): Use lookup_name.
	* decl.c (cxx_init_decl_processing): Drop global_type_node.
	* decl2.c (cplus_decl_attributes): Don't SET_IDENTIFIER_TYPE_VALUE
	here.
	* init.c (get_type_value): Delete.
	* pt.c (instantiate_class_template_1): Don't call pushtag or
	SET_IDENTIFIER_TYPE_VALUE here.
	(tsubst): Assert never an identifier.
	(dependent_type_p): Drop global_type_node assert.
	* typeck.c (error_args_num): Don't use IDENTIFIER_HAS_TYPE_VALUE
	to determine ctorness.
	gcc/testsuite/
	* g++.dg/lookup/pr99039.C: New.