[CUDA] Make CUDA compilation usable by default.
Currently clang requires several additional command
line options in order to enable new features needed
during CUDA compilation. This patch makes these
- Automatically include cuda_runtime.h if we've found a valid CUDA installation.
- Disable automatic CUDA header inclusion during unit tests.
- Added test case for command line construction.
- Enabled target overloads and relaxed call checks that are needed in order to include CUDA headers.
- Added CUDA-7.5 installation path to the CUDA installation search list.
- Define CUDA macro to indicate CUDA compilation.