Fix BTreeSet and BTreeMap gdb pretty-printers
The BTreeSet and BTreeMap gdb pretty-printers did not take the node
structure into account, and consequently only worked for shallow sets.
This fixes the problem by iterating over child nodes when needed.
This patch avoids the current approach of implementing some of the
value manipulations in debugger-indepdendent code. This was done for
convenience: a type lookup was needed for the first time, and there
currently are no lldb formatters for these types.
Closes#55771
ed6c7b751d