[clangd] Fix toHalfOpenFileRange where start/end endpoints are in different files due to #include

Summary: https://github.com/clangd/clangd/issues/129

Reviewers: SureYeaah

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66590

llvm-svn: 370029