Bug 262310, Bug 262311, Bug 262318, and Bug 262331 involve changes to FEATURE_DEFINES and how the values there relate to those found in the Platform*.h files. Those changes break XCBuild (by removing the .xcfilelist related to UnifiedSources and the process for generating them), and so are being reverted.
<rdar://problem/63893033>
Created attachment 400856 [details] Patch
Comment on attachment 400856 [details] Patch rs=me. it also seems ok to commit this unreviewed since it fixes the build.
I thought it a good idea to pass these changes by others since I was reverting their changes without completely understanding them. But I'll keep your advice in mind for the future.
Committed r262462: <https://trac.webkit.org/changeset/262462> All reviewed patches have been landed. Closing bug and clearing flags on attachment 400856 [details].