Avoid some string allocations in JSStringJoiner::join(), in the common case where the StringViewWithUnderlyingString's view covers the whole underlying string.
Pull request: https://github.com/WebKit/WebKit/pull/12056
Committed 262228@main (72ed5e715480): <https://commits.webkit.org/262228@main> Reviewed commits have been landed. Closing PR #12056 and removing active labels.
<rdar://problem/107323654>