[llvm] Add a LLVM_API_VERSION define which is incremented in llvm every time there is a public api break. This allows the llvm backend to compile with different versions of LLVM.
[llvm] Update the LLVM backend to the LLVM mono-2014-04-17-cc117f4ad24a7ae6743ab6f08b8f5e8fc65d2d7f branch.