[clangd] Support multifile edits as output of Tweaks

Summary:
First patch for propogating multifile changes from tweak outputs to LSP
WorkspaceEdits.

Uses SM to convert tooling::Replacements to TextEdits.
Errors out if there are any inconsistencies between the draft version and the
version generated the edits.

Reviewers: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66637

llvm-svn: 371392