c++: Name as_base type

c++: Name as_base type

The as-base type never got a name.  For modules I needed to give it a
name to serialize properly, and it's useful when debugging the
compiler, so we may as well have it on trunk.  There's also a bug
where its fields can have NSDMIs from the main class.  This happens to
be silent on trunk, but can be a GC leak where we retain a deferred
parse node there. (On modules it blows up, because we're not prepared
to serialize deferred parse nodes, as they should never survive
parsing.

	gcc/cp/
	* cp-tree.h (enum cp_tree_index): Add CPTI_AS_BASE_IDENTIFIER.
	(as_base_identifier): Define.
	* decl.c (initialize_predifined_identifiers): Initialize as_base
	identifier.
	* class.c (layout_class_type): Name the as-base type.  Zap
	NSDMI its fields may have.