c++: Fix unordered entity array [PR 98843]

c++: Fix unordered entity array [PR 98843]

A couple of module invariants are that the modules are always
allocated in ascending order and appended to the module array.  The
entity array is likewise ordered, with each module having spans in
that array in ascending order.  Prior to header-units, this was
provided by the way import declarations were encountered.  With
header-units we need to load the preprocessor state of header units
before we parse the C++, and this can lead to incorrect ordering of
the entity array.  I had made the initialization of a module's
language state a little too lazy.  This moves the allocation of entity
array spans into the initial read of a module, thus ensuring the
ordering of those spans.  We won't be looking in them until we've
loaded the language portions of that particular module, and even if we
did, we'd find NULLs there and issue a diagnostic.

	PR c++/98843
	gcc/cp/
	* module.cc (module_state_config): Add num_entities field.
	(module_state::read_entities): The entity_ary span is
	already allocated.
	(module_state::write_config): Write num_entities.
	(module_state::read_config): Read num_entities.
	(module_state::write): Set config's num_entities.
	(module_state::read_initial): Allocate the entity ary
	span here.
	(module_state::read_language): Do not set entity_lwm
	here.
	gcc/testsuite/
	* g++.dg/modules/pr98843_a.C: New.
	* g++.dg/modules/pr98843_b.H: New.
	* g++.dg/modules/pr98843_c.C: New.