Invent nodetag_only attribute for Nodes.

This allows explaining gen_node_support.pl's handling of execnodes.h
and some other input files as being a shortcut for explicit marking
of all their node declarations as pg_node_attr(nodetag_only).
I foresee that someday we might need to be more fine-grained about
that, and this change provides the infrastructure needed to do so.
For now, it just allows removal of the script's klugy special case
for CallContext and InlineCodeBlock.

Discussion: https://postgr.es/m/75063.1657410615@sss.pgh.pa.us

src/include/nodes/nodes.h

@@ -61,12 +61,17 @@ typedef enum NodeTag
  *
  * - no_read: Does not support nodeRead() at all.
  *
+ * - nodetag_only: Does not support copyObject(), equal(), outNode(),
+ *   or nodeRead().
+ *
  * - special_read_write: Has special treatment in outNode() and nodeRead().
  *
  * Node types can be supertypes of other types whether or not they are marked
  * abstract: if a node struct appears as the first field of another struct
  * type, then it is the supertype of that type.  The no_copy, no_equal, and
  * no_read node attributes are automatically inherited from the supertype.
+ * (Notice that nodetag_only does not inherit, so it's not quite equivalent
+ * to a combination of other attributes.)
  *
  * Valid node field attributes:
  *