In Trac #11948 I added the warning
    -Wsimplifiable-class-constraints
which warns if the class constraints in a type signature are
simplifiable.

But in fact the fragility it warns about only happens with
NoMonoLocalBinds, so this patch switches off the warning if
you have MonoLocalBinds (and suggests using it in the error
message).

See Note [Simplifiable given constraints] in TcValidity.