Add flag --always-use-string-forwarding-table to always use the
forwarding table (usually only used for shared strings) instead of
ThinString migrations initially (during GC strings will be migrated
to normal ThinStrings). The goal is to get more coverage of this code
that is designed for shared strings.

Bug: v8:12007
Change-Id: I7eb2e5ccf0018c4ac349611aebe337d8288de5c8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3536650Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Shu-yu Guo <syg@chromium.org>
Commit-Queue: Patrick Thier <pthier@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80206}