[C++11] Add unit tests for OwningPtr<T> in preparation for changes to make

it interoperate (minimally) with std::unique_ptr<T>. This is part of my
plan to migrate LLVM to use std::unique_ptr with a minimal impact on
out-of-tree code.

Patch by Ahmed Charles with some minor cleanups (and bool casts) by me.

llvm-svn: 202608