[SE] Remove Platform*Handle classes

Summary:
As pointed out by jprice, these classes don't serve a purpose. Instead,
we stay consistent with the way memory is managed and let the Stream and
Kernel classes directly hold opaque handles to device Stream and Kernel
instances, respectively.

Reviewers: jprice, jlebar

Subscribers: parallel_libs-commits

Differential Revision: https://reviews.llvm.org/D24213

llvm-svn: 280719