Fix DROP INDEX CONCURRENTLY IF EXISTS.

This threw ERROR, not the expected NOTICE, if the index didn't exist.
The bug was actually visible in not-as-expected regression test output,
so somebody wasn't paying too close attention in commit
8cb5365.
Per report from Brendan Byrd.

Author: tglsfdc

src/backend/parser/gram.y

@@ -4941,7 +4941,7 @@ DropStmt:	DROP drop_type IF_P EXISTS any_name_list opt_drop_behavior
 				{
 					DropStmt *n = makeNode(DropStmt);
 					n->removeType = OBJECT_INDEX;
-					n->missing_ok = FALSE;
+					n->missing_ok = TRUE;
 					n->objects = $6;
 					n->arguments = NIL;
 					n->behavior = $7;

src/test/regress/expected/create_index.out

@@ -2321,7 +2321,7 @@ Indexes:
 --
 DROP INDEX CONCURRENTLY "concur_index2";				-- works
 DROP INDEX CONCURRENTLY IF EXISTS "concur_index2";		-- notice
-ERROR:  index "concur_index2" does not exist
+NOTICE:  index "concur_index2" does not exist, skipping
 -- failures
 DROP INDEX CONCURRENTLY "concur_index2", "concur_index3";
 ERROR:  DROP INDEX CONCURRENTLY does not support dropping multiple objects