Cleanup WebCore.xcodeproj a bit; add some missing derived sources, move some source files to the right groups
<rdar://problem/103403928>
Pull request: https://github.com/WebKit/WebKit/pull/7691
Committed 257957@main (41d9bc21542c): <https://commits.webkit.org/257957@main> Reviewed commits have been landed. Closing PR #7691 and removing active labels.