[sanitizer] add #ifdefs around slow unwinder to fix OS X debug build

A recent change in slow unwinder causes the OS X build to fail when using COMPILER_RT_DEBUG=On. Let's fix this by #ifdef'ing it with #if SANITIZER_CAN_SLOW_UNWIND.

See http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150622/284294.html

Reviewed at http://reviews.llvm.org/D10870

llvm-svn: 241261