Use the correct namespace for looking up matching operator!= (#68922)

`S.getScopeForContext` determins the **active** scope associated with
the given `declContext`.
This fails to find the matching `operator!=` if candidate `operator==`
was found via ADL since that scope is not active.

Instead, just directly lookup using the namespace decl of `operator==`

Fixes #68901

(cherry picked from commit 93302611)