[clangd] Config: compute config in TUScheduler and BackgroundIndex

Summary:
ClangdServer owns the question of exactly which config to create, but
TUScheduler/BackgroundIndex control threads and so decide at which point
to inject it.

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83095