[ClangD] Refactor clangd into separate components

Summary: Major refactoring to split LSP implementation, Clang API calls and threading(mostly synchronization)

Reviewers: bkramer, krasimir

Reviewed By: bkramer

Subscribers: cfe-commits, mgorny, klimek

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D33047

llvm-svn: 303067