JSCBuiltins.cpp is not a header and so doesn't need to be in the Copy Headers phase. Checking its history, it seems to have been added accidentally at the same time that JSCBuiltins.h was added.
<rdar://problem/49952133>
Created attachment 367561 [details] Patch
Comment on attachment 367561 [details] Patch Clearing flags on attachment: 367561 Committed r244385: <https://trac.webkit.org/changeset/244385>
All reviewed patches have been landed. Closing bug.