Implement IF NOT EXIST for CREATE INDEX.

Fabrízio de Royes Mello, reviewed by Marti Raudsepp, Adam Brightwell and me.

src/include/nodes/parsenodes.h

@@ -2256,6 +2256,7 @@ typedef struct IndexStmt
 	bool		deferrable;		/* is the constraint DEFERRABLE? */
 	bool		initdeferred;	/* is the constraint INITIALLY DEFERRED? */
 	bool		concurrent;		/* should this be a concurrent index build? */
+	bool		if_not_exists;	/* just do nothing if index already exists? */
 } IndexStmt;
 
 /* ----------------------