Narrow the warning for simplifiable constraints
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.
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