[clangd] Added async API to run code completion.

Summary:
ClangdServer now provides async code completion API.
It is still used synchronously by ClangdLSPServer, more work is needed
to allow processing other requests in parallel while completion (or
any other request) is running.

Reviewers: klimek, bkramer, krasimir

Reviewed By: klimek

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D38583

llvm-svn: 314989