[clangd] Support UTF-32 (i.e. codepoint) offsets.

Summary:
(Changes to UTF-8/UTF-16 here are NFC, moving things around to make the
cases more symmetrical)

Reviewers: ilya-biryukov

Subscribers: ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59927

llvm-svn: 357173