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

[kadircet]

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.

[ilya-biryukov]

LGTM.
NIT about a description: we can't be 100% certain that it's related to hyper-threading, so I'd avoid putting that to the description, maybe simply mention that this avoids higher latency for foreground operations?