Fixed AstRewriter set comprehension logic

[Zayat]

The original code creates an empty dictionary {} and invokes add to add the elements, but 'dict' objects have no attribute 'add'.
Fix: create an empty set set().

Before/After Testing

Setup:

$ cd PolyCruise/PyComponent
$ echo -e "test = {i for i in [1,2,3,4,5,5,5,6]}\nprint(test)" > test_set_comprehension.py

Before:

$ python pyinspect.py -c test_set_comprehension.py
> assert(t.elts) # should be at least one element
> AssertionError

After:

$ python pyinspect.py -c test_set_comprehension.py
$ python Temp/test_set_comprehension.py
> {1, 2, 3, 4, 5, 6}