plugins/common/muxing: split `compare_clock_snapshots_and_clock_classes()`

plugins/common/muxing: split `compare_clock_snapshots_and_clock_classes()`

A subsequent commit will want to re-use the code to compare two clock
classes.  Split `compare_clock_snapshots_and_clock_classes()` in two, to
make the comparing clock classes part independent from the comparing
clock snapshots part.

Change-Id: I7888d7d3d1339ccb6c27054919349745cb7875eb
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12053
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>