The only two places where these Forgive methods were called
was in ParseAssignmentExpression just at the time we were
calling Accumulate(). So instead of Forgiving, we can simply
not accumulate the bits that would have been forgiven.

Also slightly restructures the nearby code in
ParseAssignmentExpression, and removes the use of non-const
references in ExpressionClassifier.

Review-Url: https://codereview.chromium.org/2267223002
Cr-Commit-Position: refs/heads/master@{#38839}