[clangd] Fix background index not triggering on windows due to case mismatch.

Summary:
This isn't a general fix to all paths where we assume case-sensitivity, it's
a minimally-invasive fix targeting the llvm 9 branch.

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65320

llvm-svn: 367112