Warn when sub-classes redefine fields

[Hackerpilot]

Example of code that DMD silently accepts with warnings enabled:

class Super
{
    int x;
}
class Sub : Super
{
    int x;
}

[helikopterodaktyl]

+1. I just got bitten by this.

[WebFreak001]

@helikopterodaktyl use reactions pls so it's sortable in the issue search: https://github.com/dlang-community/D-Scanner/issues?q=sort%3Areactions+is%3Aopen