Fix: bt_stream_class_map_clock_class(): copy field type when mapping

Fix: bt_stream_class_map_clock_class(): copy field type when mapping

Issue
=====
When a CTF writer stream class's packet context type contains the
special field types `timestamp_begin`, `timestamp_end`, and `timestamp`,
they are automatically mapped to the stream class's clock's class when
the stream class is about to be frozen in bt_trace_add_stream_class().
However, because the field type is mapped as is without a prior copy, it
is possible that this field type is shared with other locations in the
metadata tree which should not be mapped to a clock class.

Solution
========
Before automatically mapping any field type to a clock class, or more
generally, before modifying any field type, make a copy and replace the
original within its parent.

Known drawbacks
===============
None.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>