[sanitizer] enable random shuffling the memory chunks inside the allocator, under a flag. Set this flag for the scudo allocator, add a test.

llvm-svn: 279793
diff --git a/compiler-rt/lib/scudo/scudo_allocator.cpp b/compiler-rt/lib/scudo/scudo_allocator.cpp
index b5d8fc8..d5284a5 100644
--- a/compiler-rt/lib/scudo/scudo_allocator.cpp
+++ b/compiler-rt/lib/scudo/scudo_allocator.cpp
@@ -38,6 +38,8 @@
   static const uptr kMetadataSize = 0;
   typedef DefaultSizeClassMap SizeClassMap;
   typedef NoOpMapUnmapCallback MapUnmapCallback;
+  static const uptr kFlags =
+      SizeClassAllocator64FlagMasks::kRandomShuffleChunks;
 };
 
 typedef SizeClassAllocator64<AP> PrimaryAllocator;