[clangd] Do not show namespace comments.

Summary:
Comments from namespaces that clangd produces are too noisy and often
not useful.

Namespaces have too many redecls and we don't have a good way of
determining which of the comments are relevant and which should be
ignored (e.g. because they come from code generators like the protobuf
compiler).

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: ioeric, MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D48211

llvm-svn: 335718