[clangd] ClangdServer::codeComplete return CodeCompleteResult, not LSP struct.

Summary:
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)

Reviewers: ioeric

Subscribers: ilya-biryukov, MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D48821

llvm-svn: 336094