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

[Utkarsh Saxena via cfe-commits]

usx95 wrote:

> Does this work for function-scope operator declarations?

@zygoloid I am not sure I follow. Could you please give an example. If this is about member operator, the search scope is class scope of the first operand.

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