[clangd] Handle workspace/didChangeWatchedFiles

Summary:
The client can send notifications when it detects watched files have
changed. This patch adds the protocol handling for this type of notification.
For now, the notification will be passed down to the ClangdServer, but it will
not be acted upon. However, this will become useful for the indexer to react
to file changes.
The events could also potentially be used to invalidate other caches
(compilation database, etc).

This change also updates the VSCode extension so that it sends the events.

Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>

Reviewers: ilya-biryukov, Nebiroth

Subscribers: ilya-biryukov

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D38422

llvm-svn: 314693