Now that $ is constant-folded correctly during the optimize step, the limited
constant-folding in dsymbol can be removed. It had the effect of creating
__dollar=0 in zombie cases where constant folding wasn't quite possible.
A big mess.

Here's a very simple new scheme: __dollar is still initialized for tuples, but
for arrays, it is ALWAYS void-initialized. Later, it might be turned into an
integer in the optimize step; otherwise, it becomes a runtime value.

The implementation now shares most of the same code as array variable
assignment.

This caused a foreach CTFE regression, and also refused code like
int[]a=null; int [] b = a[0..$];