[SE] Platforms return Device values

Summary:
Platforms were returning Device pointers, but a Device is now basically
just a pointer to an underlying PlatformDevice, so we will now just pass
it around as a value.

Reviewers: jlebar

Subscribers: jprice, jlebar, parallel_libs-commits

Differential Revision: https://reviews.llvm.org/D24537

llvm-svn: 281422