[clangd] Config: Index.Background

Summary:
We only support Build/Skip for now, but with 'Load' or similar as an
option for future (load existing shards but don't build new ones).

This requires creating the config for each TU on startup. In LLVM, this
is 4000 occurrences for a total of 800ms on my machine.
But together with caching from D83755 it is only 25ms.

Reviewers: kadircet

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83768