In order to avoid internal external uncached Strings, we can copy the
String at the moment of internalizing if it is an external & uncached
String.

Bug: v8:7790
Change-Id: Ie7ed287c105a127b8b4c867aab1a808265a922b7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2613029
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71977}