Just-in-time (JIT) compilation

[jonysy]

Add a graph along w/ a JIT compiler.

Design: ..

[jonysy]

discussion:

With OpenCL 2 the compilation model changed some to be much more flexible. Now there is an LLVM backend that targets the new SPIR-V intermediate representation, so any language compiler that targets LLVM should be capable of being used to target OpenCL. Other than that the model is pretty much the same but gains a step, source compiles to LLVM IR, LLVM IR compiles to SPIR-V, driver handles final compilation and execution.