[clangd] Reduce number of threads used by BackgroundIndex to number of physical cores.

Summary:
clangd is using as many threads as logical cores for BackgroundIndex
by default. We observed that it increases latency of foreground tasks.

This patch aims to change that default to number of physical cores to get rid of
that extra latency.

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: ioeric, MaskRay, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57819

llvm-svn: 353422