Starting with https://github.com/llvm/llvm-project/pull/154686 the
compressed_pair children are now wrapped in an anonymous structure.

This patch adjusts the LLDB data-formatters to support that.

Outstanding questions:
1. Should GetChildMemberWithName look through anonymous structures? That
will break users most likely. But maybe introducing a new API is worth
it? Then we wouldnt have to do this awkward passing around of
`anon_struct_index`
2. Do we support the layout without the anonymous structure? It's not
too much added complexity. And we did release that version of libc++, so
there is code out there compiled against it. But there is no great way
of testing it (some of our macOS matrix bots do test it i suppose, but
not in a targeted way). We have the layout "simulator" tests for some of
the STL types which I will adjust.

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](https://github.com/llvm/llvm-project/blob/3092b765ba0b2d20bd716944dda86ea8e4ad12e3/lldb/examples/synthetic/gnu_libstdcpp.py#L134).
Eventually, this could be ported to C++ as well.

Towards #24834.