[generator] makr as DIM overrides as virtual.

There were handful of ABI compatibility regressions that DIM overrides
are marked as final. They are intended to be kept virtual, so explicitly
mark them as virtual. Although they should be virtual (DIM specification-wise).

Author: atsushieno

tools/generator/CodeGenerator.cs

@@ -1084,7 +1084,7 @@ public void WriteMethod (Method method, TextWriter writer, string indent, CodeGe
 
 			string static_arg = method.IsStatic ? " static" : String.Empty;
 			bool is_explicit = opt.SupportDefaultInterfaceMethods && type.IsInterface && method.OverriddenInterfaceMethod != null;
-			string virt_ov = is_explicit ? string.Empty : method.IsOverride ? (opt.SupportDefaultInterfaceMethods && method.OverriddenInterfaceMethod != null ? "/*DIM override*/" : " override") : method.IsVirtual ? " virtual" : String.Empty;
+			string virt_ov = is_explicit ? string.Empty : method.IsOverride ? (opt.SupportDefaultInterfaceMethods && method.OverriddenInterfaceMethod != null ? "/*DIM override*/ virtual" : " override") : method.IsVirtual ? " virtual" : String.Empty;
 			if ((string.IsNullOrEmpty (virt_ov) || virt_ov == " virtual") && type.RequiresNew (method.AdjustedName)) {
 				virt_ov = " new" + virt_ov;
 			}