<rdar://problem/13563697>

The __NSArrayI synthetic children provider was running expressions to generate children, which is inefficient for large amounts of data
Reimplementing to use a faster algorithm

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@178729 91177308-0d34-0410-b5e6-96231b3b80d8