[clangd] codeComplete returns more structured completion items, LSP. NFC.

Summary:
LSP has some presentational fields with limited semantics (e.g. 'detail') and
doesn't provide a good place to return information like namespace.

Some places where more detailed information is useful:
 - tools like quality analysis
 - alternate frontends that aren't strictly LSP
 - code completion unit tests

In this patch, ClangdServer::codeComplete still return LSP CompletionList, but
I plan to switch that soon (should be a no-op for ClangdLSPServer).

Deferring this makes it clear that we don't change behavior (tests stay the
same) and also keeps the API break to a small patch which can be reverted.

Reviewers: ioeric

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

Differential Revision: https://reviews.llvm.org/D48762

llvm-svn: 335980