[clangd] Check file path of declaring header when deciding whether to insert include.

Summary:
Previously, we would use include spelling of the declaring header to check
whether the inserted header is the same as the main file. This doesn't help because
we only have file path of the main file.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60687

llvm-svn: 358496