[clangd] Make functions of ClangdServer callback-based

Summary:
As a consequence, all LSP operations are now handled asynchronously,
i.e. they never block the main processing thread. However, if
-run-synchronously flag is specified, clangd still runs everything on
the main thread.

Reviewers: sammccall, ioeric, hokein

Reviewed By: sammccall, ioeric

Subscribers: klimek, jkorous-apple, cfe-commits

Differential Revision: https://reviews.llvm.org/D43227

llvm-svn: 325233