Source formatting for Swift

[wilhuff]

scripts/style.sh is based on clang-format and we have it set up to format our Objective-C, Objective-C++, and C++ code. Extending this to format .swift files broke them horribly.

SwiftFormat looks promising. We should integrate it into scripts/style.sh for swift sources.

[ryanwilson]

There's a bunch of work being done right now on libSyntax that we should be able to take advantage of, too.