[clangd] Support incremental document syncing

Summary:
This patch adds support for incremental document syncing, as described
in the LSP spec.  The protocol specifies ranges in terms of Position (a
line and a character), and our drafts are stored as plain strings.  So I
see two things that may not be super efficient for very large files:

- Converting a Position to an offset (the positionToOffset function)
  requires searching for end of lines until we reach the desired line.
- When we update a range, we construct a new string, which implies
  copying the whole document.

However, for the typical size of a C++ document and the frequency of
update (at which a user types), it may not be an issue.  This patch aims
at getting the basic feature in, and we can always improve it later if
we find it's too slow.

Signed-off-by: Simon Marchi <simon.marchi@ericsson.com>

Reviewers: malaperle, ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: MaskRay, klimek, mgorny, ilya-biryukov, jkorous-apple, ioeric, cfe-commits

Differential Revision: https://reviews.llvm.org/D44272

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@328500 91177308-0d34-0410-b5e6-96231b3b80d8
10 files changed