Native integer types from source should be preserved on override

[333fred]

Today we copy over tuple and dynamic modifiers from source on override, but we do not copy over native integer modifiers. This causes an assert to fire in the compiler for this code:

class Base { public virtual nint[] M1() => throw null; }
class Derived : Base { public override System.IntPtr[] M1() => throw null; }

It should probably warn or error.