[clangd] Use accessible scopes to query indexes for global code completion.

Summary:
 * For qualified completion (foo::a^)
       * unresolved qualifier - use global namespace ("::")
       * resolved qualifier - use all accessible namespaces inside the resolved qualifier.
 * For unqualified completion (vec^), use scopes that are accessible from the
      scope from which code completion occurs.

Reviewers: sammccall, ilya-biryukov

Reviewed By: sammccall

Subscribers: jkorous-apple, ioeric, klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D42073

llvm-svn: 323189