checking: avoid verify_type_variant crash on incomplete type.

Here, we end up calling gen_type_die_with_usage for a type that's in the
middle of finish_struct_1, after we set TYPE_NEEDS_CONSTRUCTING on it but
before we copy all the flags to the variants--and, significantly, before we
set its TYPE_SIZE.  It seems reasonable to only look at
TYPE_NEEDS_CONSTRUCTING on complete types, since we aren't going to try to
create an object of an incomplete type any other way.

	PR c++/92601
	* tree.c (verify_type_variant): Only verify TYPE_NEEDS_CONSTRUCTING
	of complete types.