Also insert NestedVariableDeclarations in the preparser if they occur. This
should be uncommon enough to not hurt preparser performance. This will also
allow us to stop checking for conflicts on already preparsed code. Since the
preparser itself will mainly run off the main thread, this can allow us to free
some main-thread time.

Bug: v8:7829, v8:8706
Change-Id: I03f2690eb7b22e941995d6f2697e64211ddbeffb
Reviewed-on: https://chromium-review.googlesource.com/c/1430069Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59044}