Fix range length comparison in DraftStore::UpdateDraft when Unicode characters...

Fix range length comparison in DraftStore::UpdateDraft when Unicode characters are removed from the document

Summary:
See http://lists.llvm.org/pipermail/clangd-dev/2018-October/000171.html for context.

I kept the error (instead of downgrading to a log message) since the range lengths differing does indicate either a bug in the client or server range calculation or the buffers being out of sync (which both seems serious enough to me to be an error). If any existing clients aside from VSCode break they should only break when accidentally typing a Unicode character which should only be a minor nuisance for a little while until the bug is fixed in the respective client.

Patch by Daan De Meyer!

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: ilya-biryukov, ioeric, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D53527

llvm-svn: 345020