Remove underscore from _Set<T>
Remove underscore from _Set<T>

rdar://problem/19132138

Make Set<T> visible by removing the underscore. Also, remove the pesky
${_Self} gyb variable that was for a temporary convenience in hiding Set.

Swift SVN r23699

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