This allows the syntax `try {} catch {}` (with no binding after the
`catch`).

See https://github.com/michaelficarra/optional-catch-binding-proposal/

Currently behind --harmony-optional-catch-binding.

As part of the implementation, this allows TryCatchStatements to not
have an associated catch scope; various paths which assumed they
would have been updated to handle this case.

Cq-Include-Trybots: master.tryserver.v8:v8_linux_noi18n_rel_ng
Change-Id: Ic525b45199eef025eb05da562e10fbd4f3d7465f
Reviewed-on: https://chromium-review.googlesource.com/571453Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Kevin Gibbons <bakkot@gmail.com>
Cr-Commit-Position: refs/heads/master@{#48300}