The new recommended toolchain for testing SourceKit-LSP is
swift-DEVELOPMENT-SNAPSHOT-2019-02-14-a (https://github.com/apple/sourcekit-lsp#recommended-toolchain). 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.
Note on clangd: We have clangd in the toolchain now, but unfortunately it's hitting frequent assertion failures during editing. I'll post an update once it's stable enough to start using.