Adds support for enqueuing parallel parse / compile tasks for eagerly
compiled IIFEs during parsing. If the --parallel-compile-tasks flag is
enabled, the parser will pre-parse eager top-level IIFEs and enqueue a
task on the compiler dispatcher to do the actual parsing / compilation
on a worker thread.

Currently we always enqueue the task, but we likely want to only
enqueue parallel tasks where the script has multiple IIFEs or a
substantial amount of top-level script code before the IIFE to avoid
the main thread having to immediately block on the parallel task. This
work will be done as a follow-up.

BUG=v8:8041

Change-Id: If68d7c374548cabd4ec32f1fb6752da7d6aaae6b
Reviewed-on: https://chromium-review.googlesource.com/c/1275354Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56593}