This patch introduces a mechanism for changing the scope of temporary
variables, which is necessary for rewriting arrow parameter
initializers.

It also fixes a potential bug in AstExpressionVisitor, which did not
visit the automatically generated members of ForEachStatement.

Fixes test/mjsunit/harmony/regress/regress-4658.js

R=rossberg@chromium.org
BUG=v8:4658
LOG=N

Review URL: https://codereview.chromium.org/1564343002

Cr-Commit-Position: refs/heads/master@{#33183}