Split the LLVM stuff out of RuntimeOptimizer to form OSL_LLVM_JITer
More RuntimeOptimizer refactor -- pull the common bits out of
RuntimeOptimizer & OSL_LLVM_JITer, to form a common base class
OSOProcessorBase.
Make OSL_LLVM_JITer a direct subclass of OSOProcessorBase, not RuntimeOptimizer
Rename OSL_LLVM_JITer to BackendLLVM