[clangd] Bail out early if we are sure that the symbol is used outside of the file.

Summary:
This would reduce the false positive when the static index is in an
unavailable state, e.g. background index is not finished.

Reviewers: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68325

llvm-svn: 373444