[StreamExecutor] Dev handles in platform interface

Summary:
This is the first in a series of patches that will convert
GlobalDeviceMemory to own its device memory handle. The first step is to
remove GlobalDeviceMemoryBase from the PlatformInterface interfaces and
use raw handles there instead. This is useful because
GlobalDeviceMemoryBase is going to lose its importance in this process.

Reviewers: jlebar

Subscribers: jprice, parallel_libs-commits

Differential Revision: https://reviews.llvm.org/D24114

llvm-svn: 280401