<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

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@178601 91177308-0d34-0410-b5e6-96231b3b80d8