[GPUJIT] Teach GPUJIT to use a pre-existing CUDA context if available.

On mixing the driver and runtime APIs, it is quite possible that a
context already exists due to runtime API usage. In this case, Polly should
try to use the same context.

This patch teaches GPUJIT to detect that a context exists and how to
pick up this context.

Without this, calling `cudaMallocManaged`, for example, before a
polly-generated kernel launch causes P100 to *hang*.

This is a part of (https://reviews.llvm.org/D35991) that was extracted
out.

Differential Revision: https://reviews.llvm.org/D36162

llvm-svn: 309802