[CAS] Integrate BitstreamWriter with CAS

[SanjayMarreddi]

This PR begins integrating the CAS into the LLVM Bitstream file format.

• We add the draft version of BitstreamCASWriter here. This is implemented by taking the existing code of BitstreamWriter. Once we agree on the approach, we will have a common base class and different implementations which has CAS or the traditional file system.

[cachemeifyoucan]

Added some high level comments.

[cachemeifyoucan]

I think this is a decent enough first iteration. I suggest to explore Reader a bit and then start to add more complicated use cases.