This replaces the AstVisitor approach for scope rewriting with a Scope-only
solution, using a new Scope::Snapshot object that keeps track of inner scopes,
unresolved variables, and temps.

The only use of the AstVisitor is now for parameter varblock scopes introduced
due to sloppy eval in parameters, which greatly simplifies the rewriter
as it no longer needs to handle temps. A future CL may be able to
eliminate it altogether by taking a snapshot per function argument.

Based on verwaest's https://codereview.chromium.org/2166023002/.

BUG=v8:5226

Review-Url: https://codereview.chromium.org/2171703004
Cr-Commit-Position: refs/heads/master@{#37989}