The new recommended toolchain for testing SourceKit-LSP is swift-DEVELOPMENT-SNAPSHOT-2018-12-02-a (https://github.com/apple/sourcekit-lsp#recommended-toolchain ). Note: for some reason this shows up with the date "Dec 1" on the website, but the link is correct. We generally expect the latest snapshot toolchain to work going forward, but until sourcekit-lsp is tested in Swift CI we will continue to recommend a specific version.
Using this toolchain along with the latest sourcekit-lsp adds support for SwiftPM packages using SE-0236 Platform Deployment Settings.