Instead of returning `nil` to indicate that the position conversion failed, log a fault and perform a best-effort recovery.
I think this allows us to perform better recovery and also makes code calling these position conversions a lot simpler because it doesn’t need to make decisions about what to do if position conversions fail.
The naming was quite inconsistent here. Let’s rename these to `LanguageService` to highlight that they belong together.
- ToolchainLanguageServer -> LanguageService
- SwiftLanguageServer -> SwiftLanguageService
- ClangLanguageServerShim -> ClangLanguageService
