Issue 3449 - const and invariant struct members do not behave according to spec 

http://d.puremagic.com/issues/show_bug.cgi?id=3449
The const/immutable members of struct/class are not manifest constant, but they are fields.
Original dmd problems are:
  1. VarDeclaration that has const or immutable type is not append as a field.
  2. Optimizer expands const/immutable field in compile time.

This fix also requires druntime and phobos changes. See
<del>https://github.com/D-Programming-Language/druntime/pull/25</del> (merged)
<del>https://github.com/D-Programming-Language/phobos/pull/84</del> (merged)
https://github.com/D-Programming-Language/phobos/pull/965