Drop @_downgrade_exhaustivity_check attribute
Drop @_downgrade_exhaustivity_check attribute

This hack was only needed for Swift 3 mode in a narrow case.  Now that
we're dropping Swift 3 from master, the source break this was working
around is no longer relevant.

apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch