[LLDB] Add formatters for MSVC STL unordered containers (#149519)
Adds formatters for MSVC STL's unordered containers. This one is
relatively simple, because it can reuse the `std::list` synthetic
children. The unordered containers (aka
[`_Hash`](https://github.com/microsoft/STL/blob/313964b78a8fd5a52e7965e13781f735bcce13c5/stl/inc/xhash#L327))
contain a
[`_List`](https://github.com/microsoft/STL/blob/313964b78a8fd5a52e7965e13781f735bcce13c5/stl/inc/xhash#L2012)
which contains all elements (and is used for iterating through the
container).
Towards https://github.com/llvm/llvm-project/issues/24834.