[lldb] Update regex to be less fragile in TestDataFormatterGenericUnordered

Follow up to D129386 where libc++ naming conventions were made consistent.

This changes the pattern to not rely on the internal name (`__cc` or `__cc_`),
and instead uses a pattern to check that the child has the form:

```
[0] = {
  first = ...
```

Thanks to @rupprecht for pointing out this issue: https://reviews.llvm.org/D133259#3773120

Reviewed By: rupprecht

Differential Revision: https://reviews.llvm.org/D133395