<rdar://problem/15449837>

Change the NSSet data formatter to not use the expression parser to produce synthetic children
In small-scale experimentation with lldb-perf, this improves our performance by around 25%



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@195294 91177308-0d34-0410-b5e6-96231b3b80d8