python: Add clear() to gdb.Record.

This function allows to clear the trace data from python, forcing to
re-decode the trace for successive commands.
This will be used in future ptwrite patches, to trigger re-decoding when
the ptwrite filter changes.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Markus Metzger <markus.t.metzger@intel.com>