[SE] GlobalDeviceMemory owns its handle

Summary:
Final step in getting GlobalDeviceMemory to own its handle.

* Make GlobalDeviceMemory movable, but no longer copyable.
* Make Device::freeDeviceMemory function private and make
  GlobalDeviceMemoryBase a friend of Device so GlobalDeviceMemoryBase
  can free its memory in its destructor.
* Make GlobalDeviceMemory constructor private and make Device a friend
  so it can construct GlobalDeviceMemory.
* Remove SharedDeviceMemoryBase class because it is never used.
* Remove explicit memory freeing from example code.

This change just consumes any errors generated during device memory freeing.
The real error handling will be added in a future patch.

Reviewers: jlebar

Subscribers: jprice, parallel_libs-commits

Differential Revision: https://reviews.llvm.org/D24195

llvm-svn: 280509