[ASan] Fix an error on invalid deallocation in ASan allocator. When ASan checks if memory freed by user was indeed previously allocated, it first does an atomic write to presumed location of chunk header. This is wrong, as if the free is invalid, we may overwrite some valuable data (like other fields of the chunk header). Fix this by using atomic_compare_exchange instead.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@177710 91177308-0d34-0410-b5e6-96231b3b80d8