[clangd] Use llvm::set_thread_priority in background-index

Reviewers: gribozavr

Subscribers: krytarowski, ilya-biryukov, ioeric, MaskRay, jkorous, arphaman, jfb, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60865

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@358664 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed