[analyzer] Add rudimentary handling of AtomicExpr.

This proposed patch adds crude handling of atomics to the static analyzer.
Rather than ignore AtomicExprs, as we now do, this patch causes the analyzer
to escape the arguments. This is imprecise -- and we should model the
expressions fully in the future -- but it is less wrong than ignoring their
effects altogether.

This is rdar://problem/25353187

Differential Revision: http://reviews.llvm.org/D21667

llvm-svn: 274816