[YSQL] ENG-4954 (#1124): Enabled DROP CONSTRAINT IF EXISTS

Summary: Enabled DROP CONSTRAINT IF EXISTS in the parser and added a test in yb_feature_alter_table

Test Plan: Added test for DROP CONSTRAINT IF EXISTS in yb_feature_alter_table

Reviewers: neha

Reviewed By: neha

Subscribers: yql

Differential Revision: https://phabricator.dev.yugabyte.com/D6474

src/postgres/src/backend/parser/gram.y

@@ -2401,7 +2401,6 @@ alter_table_cmd:
 			/* ALTER TABLE <name> DROP CONSTRAINT IF EXISTS <name> [RESTRICT|CASCADE] */
 			| DROP CONSTRAINT IF_P EXISTS name opt_drop_behavior
 				{
-					parser_ybc_signal_unsupported(@1, "ALTER TABLE DROP CONSTRAINT", 1124);
 					AlterTableCmd *n = makeNode(AlterTableCmd);
 					n->subtype = AT_DropConstraint;
 					n->name = $5;

src/postgres/src/test/regress/expected/yb_feature_alter_table.out

@@ -228,12 +228,18 @@ alter table atacc1 add constraint checkb check (b < 0); -- should fail
 ERROR:  check constraint "checkb" is violated by some row
 alter table atacc1 add constraint checkb check (b > 0);
 alter table atacc1 add constraint checkb2 check (b > 10);
+alter table atacc1 add constraint checkb3 check (b > 10);
 insert into atacc1 values (5, 5, 5); -- should fail
 ERROR:  new row for relation "atacc1" violates check constraint "checkb2"
 DETAIL:  Failing row contains (5, 5, 5).
 alter table atacc1 drop constraint checkb2;
 insert into atacc1 values (5, 5, 5);
+ERROR:  new row for relation "atacc1" violates check constraint "checkb3"
+DETAIL:  Failing row contains (5, 5, 5).
 alter table atacc1 drop constraint checkb;
+alter table atacc1 drop constraint if exists checkb2;
+NOTICE:  constraint "checkb2" of relation "atacc1" does not exist, skipping
+alter table atacc1 drop constraint if exists checkb3;
 delete from atacc1 where b = 5;
 -- test rename
 alter table atacc1 rename b to e;

src/postgres/src/test/regress/sql/yb_feature_alter_table.sql

@@ -128,10 +128,13 @@ drop table test2;
 alter table atacc1 add constraint checkb check (b < 0); -- should fail
 alter table atacc1 add constraint checkb check (b > 0);
 alter table atacc1 add constraint checkb2 check (b > 10);
+alter table atacc1 add constraint checkb3 check (b > 10);
 insert into atacc1 values (5, 5, 5); -- should fail
 alter table atacc1 drop constraint checkb2;
 insert into atacc1 values (5, 5, 5);
 alter table atacc1 drop constraint checkb;
+alter table atacc1 drop constraint if exists checkb2;
+alter table atacc1 drop constraint if exists checkb3;
 delete from atacc1 where b = 5;
 
 -- test rename