Make JSContextSpecialization constant-fold import.meta loads if the
meta object has already been created.

Most of this CL was contributed by Gus Caplan.

This is a verbatim copy of CL
https://chromium-review.googlesource.com/c/v8/v8/+/2170982
which could not be landed due to the wrong email address
being used.

TBR=verwaest@chromium.org
TBR=gsathya@chromium.org

Bug: v8:7044
Change-Id: Ief45f3082dc756265904ff500305d32717071e81
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2299375Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68875}