AddUsing tweak doesn't traverse same-level anon namespaces

[kadircet]

a.cc:

namespace ns { struct Foo; struct Bar; }
namespace { using ns::Foo; }

void foo() { ns::^Foo *f; ns::^Bar *b; }

Triggering action on first point should only drop the qualifier and second one should insert the using decl into the anon namespace.

[chouzz]

I use clangd every day and very interest in it. I would like to contribute to clangd.

I've fixed this issue and added test cases, but I'm not sure if my fix is a best practice, will submit my patch later.

[chouzz]

https://reviews.llvm.org/D156441