Auto-download Linux toolchains when possible

[allevato]

The repository rule on Linux should detect if the Swift toolchain is not already on the path; if that's the case, and if the user is running one of the Ubuntu versions for which swift.org hosts prebuilt toolchains, the rule should download that repository and use the toolchain from there.

Although this doesn't prevent the user from having to install clang, ICU, and some other components externally, it still improves ramp-up time, and makes it easy to switch between toolchains.

[keith]

No near term plans to handle this.

[keith]

Reopening since this would help some cases #665

[keith]

For CI #672 is probably good enough