Support 'var' intent on gpuizables loop

[stonea]

Currently we error out (saying that var intents aren't supported for foreach) if you attempt something like this:

foreach i in 1..100 with (var x = new MyThing()) { } 

But this might be a useful way of making a thread-private variable for a gpuizable loop (which might be useful in cases where @gpu.itersPerThread is not 1).