[asan] *experimental* implementation of invalid-pointer-pair detector (finds when two unrelated pointers are compared or subtracted). This implementation has both false positives and false negatives and is not tuned for performance. A bug report for a proper implementation will follow.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202389 91177308-0d34-0410-b5e6-96231b3b80d8