[gardening] Add "-*- swift -*-" to *.swift.gyb. Remove from *.swift.
[gardening] Add "-*- swift -*-" to *.swift.gyb. Remove from *.swift.

apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation