IPC::Attachment should be movable Currently OS(DARWIN) and OS(WINDOWS) variants do not release resources correctly.
<rdar://problem/91762600>
Pull request: https://github.com/WebKit/WebKit/pull/3954
Committed 254178@main (7456250f977b): <https://commits.webkit.org/254178@main> Reviewed commits have been landed. Closing PR #3954 and removing active labels.