Fix TestDataFormatterLibcxxListLoop.py test

Summary:
The compilation of the TestDataFormatterLibcxxListLoop.py currently fails with this error:

```
functionalities/data-formatter/data-formatter-stl/libcxx/list/loop/main.cpp:19:24: error: no member named '__value_' in 'std::__1::__list_node_base<int, void *>'
    assert(third_elem->__value_ == 3);
           ~~~~~~~~~~  ^
```

It seems the internal structure of list has changed with the 3.8 release. This patch makes the test compile with the current libc++ and with the previous libc++.

Reviewers: shafik, zturner, labath

Reviewed By: labath

Subscribers: christof, jdoerfert, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D58273

llvm-svn: 354202