[System Mode] System Model posted

I finally finished up porting our system model[1] over to the monorepo.
Differential is here:

https://reviews.llvm.org/D58736

Please have a look and comment. It's a large patch and I expect to
break it up for actual commit but wanted to put something out with full
context so people can see how it would be used.

Thanks for your feedback!

                     -David

[1] https://lists.llvm.org/pipermail/llvm-dev/2018-October/127319.html