[clangd] Allow disble down traversals from root.

Summary:
This is useful for symbo scope proximity, where down traversals from
the global scope if not desired.

Reviewers: sammccall

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

Differential Revision: https://reviews.llvm.org/D53317

llvm-svn: 344604