Make if_exists and if_not_exists flags on ddl statements match compiler

Make if_exists and if_not_exists flags on ddl statements match compiler

Added ``if_exists`` and ``if_not_exists`` parameters for all "Create" /
"Drop" constructs including :class:`.CreateSequence`,
:class:`.DropSequence`, :class:`.CreateIndex`, :class:`.DropIndex`, etc.
allowing generic "IF EXISTS" / "IF NOT EXISTS" phrases to be rendered
within DDL. Pull request courtesy Jesse Bakker.

Fixes: #7354
Closes: #8492
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/8492
Pull-request-sha: d107c6ce553bd430111607815f5b3938ffc4770c

Change-Id: I367e57b2d9216f5180bcc44e86ca6f3dc794e5ca