Add support for dense CUDA solvers #2

1. Add CUDADenseQR & tests.
   CUDADenseQR uses the cuSolverDN LAPACK implementation
   of QR factorization. A key limitation, however, is that
   this solver does not perform singularity checking --
   this is because cuSolverDN does not have a trtrs
   implementation; we instead use cuBLAS' trsv for
   backsubstitution.
2. All CPU -> GPU memory transfers are now async, and both
   CUDADenseQR and CUDADenseCholesky explicitly manage their
   own streams for async operations.
3. Simplified CUDADenseCholesky to only use the legacy 32-bit
   cuSolverDN API.

Change-Id: I2a9b7b65469658ddfe33b5b2a3892c8744d6e437