[clangd] cleanup: unify the implemenation of checking a location is inside main file.

Summary: We have variant implementations in the codebase, this patch unifies them.

Reviewers: ilya-biryukov, kadircet

Subscribers: MaskRay, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64915

llvm-svn: 366541