[clangd] Always send file URIs to editors

Summary:
Editors only know about file URIs, make sure we do not use any custom
schemes while sending edits.

Reviewers: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68324

llvm-svn: 373435