Relevant issue: https://github.com/clangd/clangd/issues/1361
The problem here was that writing something like using ns::ScopedEnum
does not cause Sema to visit this scope, to it won't be added into
CodeCompletionBuilder's AccessibleScopes, leading to the qualifier not
To detect this, walk up the DeclContext to check if we have a matching
using declaration. If so, drop the qualifiers.