This provides more structured information that embedders can use for rendering.
ClangdLSPServer continues to call render(), so NFC.
The patch is:
- trivial changes to ClangdServer/ClangdLSPServer
- mostly-mechanical updates to CodeCompleteTests etc for the new API
- new direct tests of render() in CodeCompleteTests
- tiny cleanups to CodeCompletionItem (operator<< and missing initializers)