[clangd] Add more filters for collected symbols.

Summary:
o We only collect symbols in namespace or translation unit scopes.
o Add an option to only collect symbols in included headers.

Reviewers: hokein, ilya-biryukov

Reviewed By: hokein, ilya-biryukov

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D41823

llvm-svn: 322193