[CUDA] renamed cuda_runtime.h wrapper to __cuda_runtime.h

Currently it's easy to break CUDA compilation by passing
"-isystem /path/to/cuda/include" to compiler which leads to
compiler including real cuda_runtime.h from there instead
of the wrapper we need.

Renaming the wrapper ensures that we can include the wrapper
regardless of user-specified include paths and files.

Differential Revision: http://reviews.llvm.org/D15534

llvm-svn: 255802