Implement IF NOT EXIST for CREATE INDEX.

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

src/include/catalog/index.h

@@ -60,7 +60,8 @@ extern Oid index_create(Relation heapRelation,
 			 bool allow_system_table_mods,
 			 bool skip_build,
 			 bool concurrent,
-			 bool is_internal);
+			 bool is_internal,
+			 bool if_not_exists);
 
 extern void index_constraint_create(Relation heapRelation,
 						Oid indexRelationId,

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;
 
 /* ----------------------