[StreamExecutor] Pass device memory by ref

Summary:
Step 3 of getting GlobalDeviceMemory to own its handle.

Since GlobalDeviceMemory will no longer by copy-constructible, we must
pass instances by reference rather than by value.

Reviewers: jlebar

Subscribers: jprice, parallel_libs-commits

Differential Revision: https://reviews.llvm.org/D24172

llvm-svn: 280439