clock-correlation-validator: add clock correlation validation util

clock-correlation-validator: add clock correlation validation util

`lib/graph/iterator.c` and `plugins/utils/muxer/msg-iter.cpp` have
equivalent code to verify that the various clock classes they see are
correlatable.  Add a util to factor out this logic.

Since the lib is written in C and the muxer component in C++, we need
both interfaces.  I chose to write the core of the library with a proper
C++ interface (since that's the future!) and make a small C wrapper
around it.  If/when we migrate some parts of the lib to C++, we will be
able to drop this C wrapper.

Change-Id: Iab8968ef33c9f7d4e2e8e06e0942de749a090841
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11753
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>