Trim virtual method bodies if they are not called via `base` and the type itself is not constructed.

Let's say that we have the following code:

``` csharp
using System.Text.RegularExpressions;

Console.WriteLine(new Derived().Get().IsMatch("226"));

public class Base
{
public virtual Regex Get() => new Regex(@"\d+");
}

public partial class Derived : Base
{
[RegexGenerator(@"\d+")]
private static partial Regex GetRegex();

public override Regex Get() => GetRegex();
}
```

`Base.Get` is never called because first, we don't do `new Base()` anywhere and second, descendants of `Base` do not call `base.Get()`. Its method body is preserved however after trimming. The linker could recognize that and trim it away.