[clang] Use the correct namespace for looking up matching operator!= (PR #68922)

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

Oh, one last thing! @usx95 maybe add a test for the visibility check?
Various tests with modules (search for files with the `.cppm` extension in the tests) should give a good starting point.

https://github.com/llvm/llvm-project/pull/68922