[clangd] Handle exit notification (proper shutdown)

Summary:
This changes the onShutdown handler to do essentially nothing (for now), and
instead exits the runloop when we receive the exit notification from the client.

Some clients may wait on the reply from the shutdown request before sending an
exit notification. If we exit the runloop already in the shutdown request, a
client might block forever.

This also gives us the opportunity to do any global cleanups and/or
serializations of PCH preambles to disk, but I've left that out for now.

See the LSP protocol documentation for details.

Reviewers: malaperle, krasimir, bkramer, sammccall, ilya-biryukov

Reviewed By: malaperle, sammccall, ilya-biryukov

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D38939

llvm-svn: 316564