Lazy evaluation of enum members

[Walter Bright]

Enum member forward references now work:

void main()
{
     enum E
     {
         A = B,
         E = D + 7,
         B = 3,
         C,
         D,
     }

     assert(E.A == 3);
     assert(E.B == 3);
     assert(E.C == 4);
     assert(E.D == 5);
     assert(E.E == 12);
     assert(E.max == 12);
}

https://github.com/D-Programming-Language/dmd/pull/2568

Once this is pulled, I intend to extend it so that enum members for imports are 
not semantically evaluated at all unless they are used. This is a first step to 
making all semantic evaluation of imports lazy, which should give us a big boost 
in compilation speed, as well as do a much better job at handling forward 
references.

I've been meaning to do this for some time, starting with enums because they are 
the easiest.