Fix the NSPathStore2 data formatter to actually handle the explicit length stored inside the object. The meat of this commit, however, is a nice little API for easily adding new __lldb_autogen_ helper types to an AST context

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@220881 91177308-0d34-0410-b5e6-96231b3b80d8