Warn when an equality constraint comes into scope without -XMonoLocalBinds

As imagined in #20485 (comment 393506), we might have an equality given come into scope even without a pattern match:

{-# LANGUAGE TypeFamilies, RankNTypes #-}
module A where
higher :: forall a. (forall b. (a ~ F b) => ...) -> ...

import A
x = higher (\ ... -> ...)

The lambda argument to higher has an equality given in scope without MonoLocalBinds. This should produce the warning introduced by !7042 (closed).