Allow different enums as return type in CreateDelegate (case 1288796)

Allow `CreateDelegate` to have return type covariance with methods that
return `enum` or `int` types.

This is from the upstream change at mono@1224943.

Author: Josh Peterson

mcs/class/corlib/System/Delegate.cs

@@ -160,6 +160,17 @@ private static bool return_type_match (Type delReturnType, Type returnType) {
 				// Delegate covariance
 				if (!returnType.IsValueType && delReturnType.IsAssignableFrom (returnType))
 					returnMatch = true;
+				else
+				{
+					bool isDelArgEnum = delReturnType.IsEnum;
+					bool isArgEnum = returnType.IsEnum;
+					if (isArgEnum && isDelArgEnum)
+						returnMatch = Enum.GetUnderlyingType(delReturnType) == Enum.GetUnderlyingType(returnType);
+					else if (isDelArgEnum && Enum.GetUnderlyingType(delReturnType) == returnType)
+						returnMatch = true;
+					else if (isArgEnum && Enum.GetUnderlyingType(returnType) == delReturnType)
+						returnMatch = true;
+				}
 			}
 
 			return returnMatch;