[clangd] Attempt to fix failing Windows buildbots.

The assertion is failing on Windows, probably because path separator is different.

For the failure see:
http://lab.llvm.org:8011/builders/llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast/builds/28072/steps/test/logs/stdio

llvm-svn: 371422