This allocates and populates potential source position table before the
underlying {Code} objects is allocated. It essentially makes the field
holding said table immutable after allocation.

R=verwaest@chromium.org
BUG=v8:6792

Change-Id: If35462688a1b502f28ae84f73b82b5df5005735f
Reviewed-on: https://chromium-review.googlesource.com/727895Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48781}