[clangd] Don't show base class versions of members as completions.

Summary:
These are available via qualifiers, but signal to noise level is low.
Keep required quailifier machinery around though, for cross-ns completion.

Reviewers: ioeric

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

Differential Revision: https://reviews.llvm.org/D53571

llvm-svn: 345141