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.
Strategy: If the end goal is to touch e.g. `stamp.std`, then we first
touch `stamp.std.start_time` before doing anything else. Then when
the receipe finishes, we touch `stamp.std` using the timestamp from
`stamp.std.start_time` as the reference time, and remove
`stamp.std.start_time`.
Fix#6518.
