[clangd] Fix AddUsing tweak for out-of-line functions.

Summary:
We used getEnclosingNamespaceContext(), which calls getParent() rather
than getLexicalParent(), so we would end up adding the "using" line in
places that do not affect the cursor location, or just return an error
when declaration was in another file.

Patch by Adam Czachorowski!

Reviewers: hokein

Reviewed By: hokein

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79496