This patch introduces the concept of offloading tool chain and offloading kind. Each tool chain may have associated an offloading kind that marks it as used in a given programming model that requires offloading.
It also adds the logic to iterate on the tool chains based on the kind. Currently, only CUDA is supported, but in general a programming model (an offloading kind) may have associated multiple tool chains that require supporting offloading.
This patch does not add tests - its goal is to keep the existing functionality.
This patch is the first of a series of three that attempts to make the current support of CUDA more generic and easier to extend to other programming models, namely OpenMP. It tries to capture the suggestions/improvements/concerns on the initial proposal in http://lists.llvm.org/pipermail/cfe-dev/2016-February/047547.html. It only tackles the more consensual part of the proposal, i.e.does not address the problem of intermediate files bundling yet.