Xcode: Generate 'folder' source type for directories (#14498)

Teach the Xcode generator to set 'lastKnownFileType' to be 'folder' for
file references that are directories.  If you set 'explicitFileType' to
'sourcecode', then Xcode cannot browse the directory.
Merge topic 'xcode-folder-types' into next

e5ec8ad Xcode: Generate 'folder' source type for directories (#14498)