I touched on this briefly here; I don't think we have a formal definition of what components must be or is defined to be included in a toolchain, and doing that would induce defining what packages should comprise a Swift toolchain on a target system.
That being said, I don't think this proposal should deal with this, because it drags in some ancillary concerns that may not apply generally here, but answering that question probably deserves a proposal in and of itself.