When --shared-string-table is passed, in-place-internalizable strings
are promoted into the shared old space to maintain the invariant that
in-place internalization can be done without copying.

Also some drive-by comment fixes and removal of unnecessary 'explicit'
on multi-parameter constructors.

Bug: v8:12007
Change-Id: I467d865e41934b1d5cdf85cbecc85c4befbfeb21
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3193591
Auto-Submit: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77326}