visitor-generate-ir.c: automatically map specific fields to trace's clock class

visitor-generate-ir.c: automatically map specific fields to trace's clock class

In non-strict mode (default), automatically map packet context field
type's `timestamp_begin` and `timestamp_end` fields, as well as
event header field type's `timestamp` and `ts` fields, when the
following conditions are all true:

* The decoder is in non-strict mode.
* The field type is an integer field type.
* The integer field type is not mapped to a clock class.
* The trace contains only one clock class at this point.

This attempts to fix semantics errors committed by some known tracers
(very old versions of LTTng and old versions of CTF writer).

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>