<rdar://problem/15235492>

Extend DummySyntheticProvider to actually use debug-info vended children as the source of information
Make Python synthetic children either be valid, or fallback to the dummy, like their C++ counterparts

This allows LLDB to actually stop bailing out upon encountering an invalid synthetic children provider front-end, and still displaying the non synthetized ivar info

llvm-svn: 192741