[clangd] Deduplicate query scopes.

Summary:
For example, when anonymous namespace is present, duplicated namespaces might be
generated for the enclosing namespace.

Reviewers: ilya-biryukov

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

Differential Revision: https://reviews.llvm.org/D54105

llvm-svn: 346224