[CUDA][OpenMP] Create generic offload toolchains

Summary:
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.

Reviewers: ABataev, jlebar, echristo, hfinkel, tra

Subscribers: guansong, Hahnfeld, andreybokhanko, tcramer, mkuron, cfe-commits, arpith-jacob, carlo.bertolli, caomhin

Differential Revision: http://reviews.llvm.org/D18170

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@272571 91177308-0d34-0410-b5e6-96231b3b80d8