swift234

[arc] Add a new data structure called ImmutablePointerSet.

This is an immutable data structure with the following properties:

1. All of the sets are sorted and can be iterated over.
2. It takes in a bump ptr allocator and uses that allocator for all
allocations.
3. All concatenation operations involve only one bump ptr allocation.
4. Since we are only storing pointers, the data structure does not need any
destructors to be invoked to be cleaned up. The bumpptrallocator memory just
needs to be freed.

I am going to use this to improve the compile time performance of ARC.