Adds synthetic children for MSVC STL's
[`std::tuple`](313964b78a/stl/inc/tuple).
A `tuple` is a chain of base classes:
```cpp
template <>
class tuple<> {};
template <class _This, class... _Rest>
class tuple<_This, _Rest...> : private tuple<_Rest...> {
_Tuple_val<_This> _Myfirst;
};
```
So the provider walks the base classes to the desired one.
The implementation makes it hard to detect if the empty tuple is from
this STL. Fortunately, libstdc++'s synthetic children provider works for
empty MSVC STL tuples as well.
Towards #24834.
eec98bef84