Merging r367112:

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r367112 | sammccall | 2019-07-26 16:07:11 +0200 (Fri, 26 Jul 2019) | 11 lines

[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
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llvm-svn: 367135