The diagnostic group documentation now point to the swift.org URL rather
than the toolchain path, so it no longer needs to be passed all the way
through sourcekitd.
Resolves rdar://151500502.
This allows us to load plugins into the sourcekitd service to handle requests (currently only used for code completion). This allows us to implement parts of sourcekitd in Swift and outside of the compiler repository, making it easier to iterated on them because the compiler doesn’t need to be rebuilt.
