clangd: use -j for background index pool

Summary:
clangd supports a -j option to limit the amount of threads to use for parsing
TUs. However, when using -background-index (the default in later versions of
clangd), the parallelism used by clangd defaults to the hardware_parallelisn,
i.e. number of physical cores.

On shared hardware environments, with large projects, this can significantly
affect performance with no way to tune it down.

This change makes the -j parameter apply equally to parsing and background
index. It's not perfect, because the total number of threads is 2x the -j value,
which may still be unexpected. But at least this change allows users to prevent
clangd using all CPU cores.

Reviewers: kadircet, sammccall

Reviewed By: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66031

llvm-svn: 368498