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
GDB and LLDB pretty printers have some common functionality
and also access some common information, such as the layout of
standard library types. So far, this information has been
duplicated in the two pretty printing python modules. This
commit introduces a common module used by both debuggers.
