Depends on https://github.com/swiftlang/swift/pull/83378
---
Adds support for the LSP signature help request.
> [!NOTE]
> As of https://github.com/swiftlang/swift/pull/83378, SourceKitD still
doesn't separate parameter documentation from the signature
documentation and thus parameters don't have their own separate
documentation. This should just work once SourceKitD implements this
functionality and we'll only need to modify the tests.
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`.
