Fix ExprState's tag to be of type NodeTag rather than Node.

This appears to have been an oversight in b8d7f053c5c2. As it's
effectively harmless, though confusing, only fix in master.

Author: Andres Freund

src/backend/executor/execExpr.c

@@ -361,7 +361,7 @@ ExecBuildProjectionInfo(List *targetList,
 
 	projInfo->pi_exprContext = econtext;
 	/* We embed ExprState into ProjectionInfo instead of doing extra palloc */
-	projInfo->pi_state.tag.type = T_ExprState;
+	projInfo->pi_state.tag = T_ExprState;
 	state = &projInfo->pi_state;
 	state->expr = (Expr *) targetList;
 	state->parent = parent;

src/include/nodes/execnodes.h

@@ -61,7 +61,7 @@ typedef Datum (*ExprStateEvalFunc) (struct ExprState *expression,
 
 typedef struct ExprState
 {
-	Node		tag;
+	NodeTag		tag;
 
 	uint8		flags;			/* bitmask of EEO_FLAG_* bits, see above */