[clangd] Cleanup the duplicated getTokenRange.

Summary:
Also lift it to SourceCode.h, so that it can be used in other places
(semantic code highlighting).

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63714

llvm-svn: 364280