Expose CUDA function attributes to the C interface.

Until now all CUDA-specific attributes were represented with
CXCursor_UnexposedAttr; now they are actually implemented, including the Python
bindings.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209767 91177308-0d34-0410-b5e6-96231b3b80d8