[msan] Better use-after-free reports.

By attaching an extra integer tag to heap origins, we are able
to distinguish between uninits
 - created by heap allocation,
 - created by heap deallocation (i.e. use-after-free),
 - created by __msan_allocated_memory call,
 - etc.

See https://code.google.com/p/memory-sanitizer/issues/detail?id=35.

llvm-svn: 226821