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.

gcc/cp/class.c

@@ -6715,6 +6715,10 @@ layout_class_type (tree t, tree *virtuals_p)
       /* T needs a different layout as a base (eliding virtual bases
 	 or whatever).  Create that version.  */
       tree base_t = make_node (TREE_CODE (t));
+      tree base_d = create_implicit_typedef (as_base_identifier, base_t);
+
+      TYPE_CONTEXT (base_t) = t;
+      DECL_CONTEXT (base_d) = t;
 
       /* If the ABI version is not at least two, and the last
 	 field was a bit-field, RLI may not be on a byte
@@ -6751,6 +6755,9 @@ layout_class_type (tree t, tree *virtuals_p)
 	if (TREE_CODE (field) == FIELD_DECL)
 	  {
 	    *next_field = copy_node (field);
+	    /* Zap any NSDMI, it's not needed and might be a deferred
+	       parse.  */
+	    DECL_INITIAL (*next_field) = NULL_TREE;
 	    DECL_CONTEXT (*next_field) = base_t;
 	    next_field = &DECL_CHAIN (*next_field);
 	  }
@@ -6760,8 +6767,6 @@ layout_class_type (tree t, tree *virtuals_p)
 	 needs a mode.  */
       compute_record_mode (base_t);
 
-      TYPE_CONTEXT (base_t) = t;
-
       /* Record the base version of the type.  */
       CLASSTYPE_AS_BASE (t) = base_t;
     }

gcc/cp/cp-tree.h

@@ -148,6 +148,7 @@ enum cp_tree_index
     CPTI_DELTA_IDENTIFIER,
     CPTI_IN_CHARGE_IDENTIFIER,
     CPTI_VTT_PARM_IDENTIFIER,
+    CPTI_AS_BASE_IDENTIFIER,
     CPTI_THIS_IDENTIFIER,
     CPTI_PFN_IDENTIFIER,
     CPTI_VPTR_IDENTIFIER,
@@ -289,6 +290,7 @@ extern GTY(()) tree cp_global_trees[CPTI_MAX];
 /* The name of the parameter that contains a pointer to the VTT to use
    for this subobject constructor or destructor.  */
 #define vtt_parm_identifier		cp_global_trees[CPTI_VTT_PARM_IDENTIFIER]
+#define as_base_identifier		cp_global_trees[CPTI_AS_BASE_IDENTIFIER]
 #define this_identifier			cp_global_trees[CPTI_THIS_IDENTIFIER]
 #define pfn_identifier			cp_global_trees[CPTI_PFN_IDENTIFIER]
 #define vptr_identifier			cp_global_trees[CPTI_VPTR_IDENTIFIER]

gcc/cp/decl.c

@@ -4202,6 +4202,7 @@ initialize_predefined_identifiers (void)
     {"__dt_del ", &deleting_dtor_identifier, cik_dtor},
     {"__conv_op ", &conv_op_identifier, cik_conv_op},
     {"__in_chrg", &in_charge_identifier, cik_normal},
+    {"__as_base ", &as_base_identifier, cik_normal},
     {"this", &this_identifier, cik_normal},
     {"__delta", &delta_identifier, cik_normal},
     {"__pfn", &pfn_identifier, cik_normal},