[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
options default.

* 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.

llvm-svn: 253389