c++: Completeness of typedef structs [PR 99294]

c++: Completeness of typedef structs [PR 99294]

When we read in a class definition, we use fixup_type_variants to
propagate the now-completed fields of the class's TYPE to other
variants.  Unfortunately that doesn't propagate all of them, and in
this case we had a typedef to an (incomplete) instantiation.  That
typedef ended up with a VOIDmode, which blew up gimple expansion as
the type itself isn't VOID.  Without modules, that information is
propagated in finalize_type_size when laying out the class.  But that
doesn't happen with stream-in -- we already know the layout.  There is
already some overlap between the two functions, now there's a bit
more.  In fixup_type_variants, I pay attention to the TYPE_NAME to
decide whether to override a user's TYPE_ALIGN -- variants of the
main-variant typedef just copy the main-variant.  Other variants
recalculate.  Overaligning is still permitted.

I also added a TYPE_ALIGN_RAW accessor, and fixed a bug in the
alignment streaming I noticed.  I did not refactor TYPE_ALIGN beyond
using the new accessor.  (It could be written as ((1 << align_raw) >>
1), rather than use the conditional.)

	PR c++/99294
	gcc/
	* tree.h (TYPE_ALIGN_RAW): New accessor.
	(TYPE_ALIGN): Use it.
	gcc/cp/
	* class.c (fixup_type_variants): Propagate mode, precision,
	alignment & emptiness.
	* module.cc (trees_out::type_node): Use TYPE_ALIGN_RAW.
	(trees_in::tree_node): Rematerialize alignment here.
	gcc/testsuite/
	* g++.dg/modules/pr99294.h: New.
	* g++.dg/modules/pr99294_a.C: New.
	* g++.dg/modules/pr99294_b.C: New.