Fix MA0046 reporting override events

[meziantou]

What

MA0046 (Use EventHandler to declare events) skipped the interface implementations, but not the overrides, so the same event was reported on the base declaration and on each override, whose type cannot be changed:

abstract class Base
{
    public abstract event System.Action E; // MA0046, expected
}

class Derived : Base
{
    public override event System.Action E; // MA0046, unexpected
}

Expected: 1 diagnostic (on Base.E). Actual: 2 diagnostics.

Fix

The analyzer skips override events, like the interface implementations. docs/Rules/MA0046.md mentions both.

Tests

Added Override to UseEventHandlerOfTAnalyzerTests (failed before the fix). The test class passes on roslyn5.9 (16/16) and roslyn4.8 (16/16).