Fix dotnet#5531

This is a working fix for the example of dotnet#5531.
However there are still some points open before I think it's a good idea to merge this:
- Discuss the change of this proposal, is it OK to prefer overrides of methods and so ignore base implementations?
- Extend the check to the complete inheritance graph instead of a single "look-back"
- Only ignore a methinfo if the signature of both match (so respect different overloads)

@dsyme It would be great if you could have a look whether this check is allowed at this location or should appear earlier.

src/fsharp/ConstraintSolver.fs

@@ -1310,7 +1310,12 @@ and SolveMemberConstraint (csenv:ConstraintSolverEnv) ignoreUnresolvedOverload p
                   minfos 
                     // curried members may not be used to satisfy constraints
                     |> List.choose (fun minfo ->
-                          if minfo.IsCurried then None else
+                          if minfo.IsCurried then None
+                          elif (minfos |> List.exists(fun minfo1 ->
+                                    match minfo1.DeclaringTyconRef.TypeContents.tcaug_super with
+                                    | Some(TType_app(parent, _)) -> tyconRefEq g parent minfo.DeclaringTyconRef
+                                    | _ -> false)) then None
+                          else
                           let callerArgs = argtys |> List.map (fun argty -> CallerArg(argty, m, false, dummyExpr))
                           let minst = FreshenMethInfo m minfo
                           let objtys = minfo.GetObjArgTypes(amap, m, minst)