Note [Simplifiable given constraints]

A type signature like
   f :: Eq [(a,b)] => a -> b
is very fragile, for reasons described at length in GHC.Tc.Solver.Dict
Note [Instance and Given overlap].  As that Note discusses, for the
most part the clever stuff in GHC.Tc.Solver.Dict means that we don't use a
top-level instance if a local Given might fire, so there is no
fragility. But if we /infer/ the type of a local let-binding, things
can go wrong (#11948 is an example, discussed in the Note).

So this warning is switched on only if we have NoMonoLocalBinds; in
that case the warning discourages users from writing simplifiable
class constraints.

The warning only fires if the constraint in the signature
matches the top-level instances in only one way, and with no
unifiers -- that is, under the same circumstances that
GHC.Tc.Instance.Class.matchInstEnv fires an interaction with the top
level instances.  For example (#13526), consider

  instance {-# OVERLAPPABLE #