Adds synthetic providers for MSVC's `std::forward_list` and `std::list`.
It refactors `LibCxxList` to be generic over the STL type (currently
libc++ or MSVC STL). The libstdc++ synthetic providers use something
similar in Python
[here](3092b765ba/lldb/examples/synthetic/gnu_libstdcpp.py (L134)).
Eventually, this could be ported to C++ as well.
Towards #24834.
