If the index for a given file is not up-to-date, perform a syntactic scan for tests within it.
This should allow editors to run the `textDocument/tests` request while the user is editing a file to scan it for test cases.
The naming was quite inconsistent here. Let’s rename these to `LanguageService` to highlight that they belong together.
- ToolchainLanguageServer -> LanguageService
- SwiftLanguageServer -> SwiftLanguageService
- ClangLanguageServerShim -> ClangLanguageService
