`SyntacticTestIndex` queries all language services for a document for its syntactic test items. This caused `syntacticDocumentTests not implemented in DocumentationLanguageService` to be logged for every Swift file in the project. Remove the default implementation of `syntacticDocumentTests` in `LanguageService` and instead provide an implementation that returns an empty array in `DocumentationLanguageService`.
While at it, also log the files for which unimplemented methods are called on `LanguageService`. This makes it easier to dermine why they are called.
With the introduction of secondary language services, we don’t expect every language service to implement every request anymore. To simplify the addition of language services like `DocumentationLanguageService` add default implementations for methods that satisfy the following criteria:
- `SourceKitLSPServer` does not expect side effects to happen when they are called
- The method can throw or there is a reasonable default value
- It is reasonable to expect that not all language services need to implement it
This cleanly separates the responsibilities for handling documentation from those of handling Swift files. It also simplifies providing docc support for clang because we just need to implement the two `symbolGraph` methods in `ClangLanguageService` and can re-use the remaining infrastructure from `DoccLanguageService`.
This allows us to implement all of `doccDocumentation` in `DocumentationLanguageService`. `DocumentationLanguageService` will be a secondary language service for Swift files and can also provide the docc documentation support that’s currently in `SwiftLangaugeService`.
