libc++: test lock-free atomic alignment

Summary:
libc++ implements std::atomic<_Tp> using __atomic_base<_Tp> with
`mutable _Atomic(_Tp) __a_`. That member must be suitably aligned on
relevant ISAs for instructions such as cmpxchg to work properly, but
this alignment isn't checked anywhere. __atomic_base's implementation
relies on _Atomic doing "the right thing" since it's under the
compiler's control, and only the compiler knows about lock-freedom and
instruction generation. This test makes sure that the compiler isn't
breaking libc++'s expectations.

I'm looking at a few odd things in the C++ standard, and will have a few
other fixes around this area in the future.

This requires building with `-DLIBCXX_HAS_ATOMIC_LIB=True`, the test
marks the dependency as REQUIRES and won't be run without.

Reviewers: cfe-commits

Subscribers: EricWF, mclow.lists

Differential Revision: http://reviews.llvm.org/D22073

llvm-svn: 277368