Issue 10326 - Disallow 'invariant' for immutable, allow class/struct invariants without (), and later disallow usage of ()

https://issues.dlang.org/show_bug.cgi?id=10326

A year ago, #2160 was merged, and using invariant keyword for immutable was changed to error. But, the change was released in 2.064, November 5, 2013.

Should we wait to fix 10326 until November 2014?