define SWIFT_CC(swift) macro for DispatchStubs
define SWIFT_CC(swift) macro for DispatchStubs

resolve FIXME left in initial Swift3 port to Linux by replicating
definition of SWIFT_CC(swift) from swift/include/Runtime/Config.h.

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