[clangd] Support utf-8 offsets (rather than utf-16) as a protocol extension

Summary:
Still some pieces to go here: unit tests for new SourceCode functionality and
a command-line flag to force utf-8 mode. But wanted to get early feedback.

Reviewers: hokein

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58275

llvm-svn: 357102