[GPUJIT] Add GPUJIT APIs for allocating and freeing managed memory.

We introduce `polly_mallocManaged` and `polly_freeManaged` as
proxies for `cudaMallocManaged` / `cudaFree`. This is currently not
used by Polly. It is auxiliary code that is used in `COSMO`.

This is useful because `polly_mallocManaged` matches the signature of `malloc`,
while `cudaMallocManaged` does not. We introduce `polly_freeManaged` for
symmetry.

We use this in COSMO to use the unified memory feature of the newer
CUDA APIs (>= 6).

Differential Revision: https://reviews.llvm.org/D35991

llvm-svn: 309808