[clangd] Drop documentation in static index if symbols are not indexed for completion.

Summary:
This is a further optimization of r350803, we drop docs in static index for
symbols not being indexed for completion, while keeping the docs in dynamic
index (we rely on dynamic index to get docs for class members).

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

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

Differential Revision: https://reviews.llvm.org/D56539

llvm-svn: 354792