Reflection: Fix DefaultValue exception with enums (#17917)

This is related to dbcfd2f, but about enums instead of `DateTime`.

Given e.g. a generic method with a parameter `T arg = default(T)`
where `T` is the generic type parameter, if that is instantiated with
some enum as the generic type argument, then querying the default
value of `arg` using `ParameterInfo.[Raw]DefaultValue` will throw a
`FormatException`.

(The use of generics means that the C# compiler always records a
`null` constant in metadata, which isn't usually the case for enums.)

src/mscorlib/src/System/Reflection/MdConstant.cs

@@ -68,6 +68,9 @@ public static unsafe Object GetValue(MetadataImport scope, int token, RuntimeTyp
                         defaultValue = buffer;
                         break;
 
+                    case CorElementType.Class:
+                        return null;
+
                     default:
                         throw new FormatException(SR.Arg_BadLiteralFormat);
                         #endregion