Issue Description:

There should be a script in swift/utils that installs Swift in the correct location for the current platform. It should either invoke build-script to build it, or it should rely on you having already built swift and just install a named build.

On OS X, the install process should consist of generating and installing an installer package for a Toolchain in /Library/Developer/Toolchains/, just as how the snapshot download from swift.org installs one. It should either update swift-latest.xctoolchain or perhaps a separate symlink swift-development.xctoolchain. I'm not sure if it needs to generate and install the separate symbols download (possibly optional).

On other platforms, it should probably build a root that installs into /usr/local, with the install location configurable, though I'm not sure if there's another alternative that makes more sense (such as providing an xcrun-like behavior so you can have several swift's installed simultaneously and select among them with env vars or an xcrun-like tool).