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

[kubamracek]

Summary

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

[samsonov]

LGTM

[glider]

LGTM

[kubamracek]

Landed in r241261.

lib/sanitizer_common/sanitizer_stacktrace_libcdep.cc

if (max_depth == 0) {
     return;
   }
   if (max_depth == 1) {
     size = 1;
     trace_buffer[0] = pc;
     return;
   }
   if (!WillUseFastUnwind(request_fast_unwind)) {
+#if SANITIZER_CAN_SLOW_UNWIND
     if (context)
       SlowUnwindStackWithContext(pc, context, max_depth);
     else
       SlowUnwindStack(pc, max_depth);
+#else
+    UNREACHABLE("slow unwind requested but not available");
+#endif
   } else {
     FastUnwindStack(pc, bp, stack_top, stack_bottom, max_depth);
   }
 }
 
 }  // namespace __sanitizer