This WL focuses to provide SQL standard syntaxes to DROP or ALTER
constraint.

Following changes are done as part of this WL:

A Syntax Changes:

  The ALTER TABLE statement is extended in a following way to support
  new clauses,

  A.1 For DROP CONSTRAINT clause:
        alter_specification:
          ...
          DROP CONSTRAINT symbol;

        Example:
          ALTER TABLE t1 DROP CONSTRAINT `primary`;

  A.2 For ALTER CONSTRAINT clause:
        alter_specification:
          ...
          ALTER CONSTRAINT symbol [NOT] ENFORCED;

        Example:
          ALTER TABLE t1 ALTER CONSTRAINT t1_chk_1 ENFORCED;

      WL supports SQL standard syntax at this point. Support to alter
      constraints of type other than CHECK constraint is out of this
      WL scope. Error is reported if constraint of type other than
      CHECK is used with ALTER CONSTRAINT clause.

B The type of constraints in DROP CONSTRAINT / ALTER CONSTRAINT
  clauses are identified by the "name" in prepare stage of the alter
  table operation.
  Constraints of different types can have the same name because of
  separate namespaces. Such ambiguous situation are handled by reporting
  an error, forcing user to use constraint specific DROP or ALTER
  clause.

C Test cases are added to the constraints.test of main suite.

Change-Id: I96ba54f110fff21520ff0e48fcc8c7dea9dc962a