Don't warn FS3559 when obj is inferred if the obj is expected there

The new FS3559 triggers where I think it should not. Consider this:

```f#
let foo (x: obj) = ()
foo null
```

It triggers on `null`:

> A type has been implicitly inferred as 'obj', which may be unintended. Consider adding explicit type annotations. You can disable this warning by using '#nowarn "3559"' or '--nowarn:3559'.

But the type of the argument is explicitly `obj`, so it doesn't make sense that it should trigger here.

Another slightly more realistic example:

```f#
let invokeStatic (mi: System.Reflection.MethodInfo) =
mi.Invoke(null, [||])
```

`MethodInfo.Invoke` expects `obj` as its first argument, so F# inferring `obj` here should not cause a warning.