<rdar://problem/13339196>

The algorithm to access an item in a __NSArrayM was not reacting properly to deletions
The fix is to use a smarter formula that accounts for items shifting and the resulting notion of offsets in the table

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@178076 91177308-0d34-0410-b5e6-96231b3b80d8