This moves source position tables associated with WasmCode objects to be
located outside the garbage-collected heap. There now is a clear link to
the source position table from code, making the one-to-one relationship
and its lifetime explicit.

R=ahaas@chromium.org
BUG=v8:7424

Change-Id: I9d0b332732508c302ba525059ef02559f45aa2f6
Reviewed-on: https://chromium-review.googlesource.com/975565
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52178}