[clangd] Add code completion support

Summary: Adds code completion support to clangd.

Reviewers: bkramer, malaperle-ericsson

Reviewed By: bkramer, malaperle-ericsson

Subscribers: stanionascu, malaperle-ericsson, cfe-commits

Differential Revision: https://reviews.llvm.org/D31328

llvm-svn: 299421