The [lldb-aarch64-windows](https://lab.llvm.org/buildbot/#/builders/141)
buildbot failed with:
```
lld-link: error: undefined symbol: printf
>>> referenced by main.o:(main)
```
I'm assuming that's because of the use of `__builtin_printf`. In other
tests, we use `printf` form `stdio.h` and these build fine, so I added
an include and used `printf`.
Synthetic providers for collection types use a child name format of
"[N]".
This `ValueObjectSynthetic` to automatically convert child names in this
convention to the index embedded in the subscript string. With this
change, synthetic formatters for collections will only need to implement
`GetIndexOfChildWithName` or `get_child_index` for non-indexed
collection children. Some examples of non-indexed children are
`$$dereference$$` support, or "hidden" children.
The automatic conversion applies to N values that are less than the
number of children reported by the synthetic provider.
