UseOfMemberOfNullReferenceIssue: Warn that ((Foo)null).Bar() will cause a NRE

This will warn about:
```csharp
int Foo(string x) {
   if (x != null) x = null;
   return x.Length;
}
```

This relies on the null value analysis. Right now this issue is very conservative, but depending on how good the null value analysis turns out to be, we could make it a bit more aggressive later.