[StreamExecutor] Add error handling library

Summary:
Error handling in StreamExecutor is based on llvm::Error and
llvm::Expected. This CL sets up the StreamExecutor wrapper classes in
the streamexecutor namespace.

All the other StreamExecutor code makes use of this error handling code,
so this is the first CL for checking in StreamExecutor.

Reviewers: jlebar, tra

Subscribers: parallel_libs-commits

Differential Revision: https://reviews.llvm.org/D22687

llvm-svn: 277210