[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

llvm-svn: 358664