This adds synthetic child providers for `std::vector<T>` and
`std::vector<bool>` for MSVC's STL.
The structure of a `std::vector<T>` is relatively similar to libc++'s
implementation that uses `__begin` and `__end`.
`std::vector<bool>` is different. It's a `std::vector<unsigned int>`
wrapper instead of `std::vector<uint8_t>`. This makes the calculation
slightly less simple. I put a comment in the `GetChildAtIndex` to make
this clear.
- [NatVis for
`std::vector<T>`](313964b78a/stl/debugger/STL.natvis (L1193-L1205))
- [NatVis for
`std::vector<bool>`](313964b78a/stl/debugger/STL.natvis (L1167-L1179))
Towards #24834.
