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.

Signed-off-by: Daniel A. Steffen <bc823475b60dbcbd7be1bbdfe095b5f1939d5bd2@apple.com>

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