[lldb] Fix libstdc++ 11's std::unique_ptr affecting LLDB testsuite TestDataFormatterStdUniquePtr.py

libstdc++ since version 11 has a conditional compilation based on
[[no_unique_address]] availability whether one element is either
inherited or put there as a field with [[no_unique_address]].

The code comment is by teemperor.

Reviewed By: teemperor

Differential Revision: https://reviews.llvm.org/D104283