Add a case to the static contract optimizer so that contracts
generated from the `Syntax` type disappear in trusted positions.
This restores the "old" behavior for the `Syntax` type --- the behavior
before fixing the 'or/c' unsoundness in issue 598.
<https://github.com/racket/typed-racket/issues/598>
Note: this fix is temporary, until the definition of `Syntax` can
be in terms of _immutable_ vectors & boxes.
d3efa46003