Report an error during scope analysis if we're unable to find a
variable proxy for the given private field. This can happen if we try
to access a private field that was not defined or if we're outside
the class scope.

This doesn't correctly throw an early error when pre parsing a top
level function because we don't track it's variables.

Bug: v8:5368
Change-Id: I0a1193fe0ae213c0732fae5d435e150852a8d87d
Reviewed-on: https://chromium-review.googlesource.com/892093Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51082}