<rdar://problem/13563403>

Reimplemented the NSDictionary synthetic children provider for added performance.
Instead of generating pairs by running an expression, we now create a pair type using clang-level APIs and fill in a buffer with the pointers to key and value
This strategy takes the time required to dump a 10k items __NSDictionaryM from ~45s to <4s

llvm-svn: 178601