Implement graphs emulation layer

[EwanC]

Implement in the DPC++ graphs PoC at the PI level a lazy queue on top of a regular queue to emulate the deferred execution behaviour of graphs.

Please feel free to add more information here @julianmi, that single sentence probably doesn't do justice to what you've been working on, and may not even be accurate.

[julianmi]

WIP can be found here: https://github.com/reble/llvm/tree/julianmi/graph-emulation

[EwanC]

Closing as this task is completed now that #71 is merged