change prototype of functions returning nil to use ImplictlyUnwrappedOptional
change prototype of functions returning nil to use ImplictlyUnwrappedOptional

Fixes build break with 20160412a development drop by restoring !
annotation on the return types of methods that return nil.
Picked ! (not ?) to match swift/public/SDK/Dispatch/Dispatch.swift.

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