<rdar://problem/13339196>
<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: 4c4cc70b1ef44ba2b7963015e681894188cea27e@178076 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb