Synthetic providers for collection types use a child name format of
"[N]".
This `ValueObjectSynthetic` to automatically convert child names in this
convention to the index embedded in the subscript string. With this
change, synthetic formatters for collections will only need to implement
`GetIndexOfChildWithName` or `get_child_index` for non-indexed
collection children. Some examples of non-indexed children are
`$$dereference$$` support, or "hidden" children.
The automatic conversion applies to N values that are less than the
number of children reported by the synthetic provider.
This PR adds formatters for `std::map`, `std::set`, `std::multimap`,
`std::multiset` as well as their iterators. It's done in one PR because
the types are essentially the same (a tree) except for their value type.
The iterators are required because of the tests.
`MsvcStlTreeIterSyntheticFrontEnd` is based on the libc++ equivalent. As
opposed to `std::list`, there aren't that many duplicates, so I didn't
create a generic type.
For reference, the tree is implemented in
313964b78a/stl/inc/xtree.
Towards #24834.
