Improvements to the data formatters SB API:

- exposing new accessors: formats/format, ..., that allow you to iterate over all formatters
 e.g. sys_category = lldb.debugger.GetCategory("system").summary['char *']
- ensuring that C++-based synthetic children provider can at least print their description accurately, if nothing else



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@183805 91177308-0d34-0410-b5e6-96231b3b80d8