Upstream a change to MemoryHistoryASan from Sean:

Author: Sean Callanan <scallanan@apple.com>
Date:   Tue Jun 23 13:52:24 2015 -0700

    Memory history should not crash if it can't inspect its data.  Added
    error handling.
            
    <rdar://problem/21231304>

llvm-svn: 252252