This changes a few bits about how continuation counters are handled.

It introduces a new mechanism that allows removal of a continuation
range after it has been created. If coverage is enabled, we run a first
post-processing pass on the AST immediately after parsing, which
removes problematic continuation ranges in two situations:

1. nested continuation counters - only the outermost stays alive.
2. trailing continuation counters within a block-like structure are
   removed if the containing structure itself has a continuation.

R=bmeurer@chromium.org, jgruber@chromium.org, yangguo@chromium.org

Bug: v8:8381, v8:8539
Change-Id: I6bcaea5060d8c481d7bae099f6db9f993cc30ee3
Reviewed-on: https://chromium-review.googlesource.com/c/1339119Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58443}