One more change required to build the sanitizers for iOS.

Summary: _Unwind_Backtrace is not available on iOS, so we should ifdef out the posix implementations of BufferedStackTrace::SlowUnwindStack and BufferedStackTrace::SlowUnwindStackWithContext on iOS.

Reviewers: samsonov

Reviewed By: samsonov

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10696

llvm-svn: 240586