This patch implements eager expression rewriting when parsing.  It will
be used for desugaring spreads but may have other uses in the future.

We call Traits::RewriteExpression as soon as we realise that something
parsed as an expression is actually used as an expression (and not as
a pattern).  This patch adds a dummy implementation for this function,
doing no rewriting at all, and adds the trigers in the right places of
the parser.

R=rossberg@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1567603005

Cr-Commit-Position: refs/heads/master@{#33300}