ctf: Add a constant to get the currently selected field for variant types

ctf: Add a constant to get the currently selected field for variant types

Variants typically have only 1 struct fields per choice and its name is the
tag. Many possible choices may have the same fields in the struct so it should
be possible to use one lookup to check in every possible choice. This patch
adds a constant to use to get the currently selected field no matter its name.

Use case for this: network packets in LTTng traces have a network_header
variant field that contains a transport_header variant field. But the various
network_header choices have the same transport_header field. We want to be
able to query the { network_header, ANY, transport_header } field and not, as
previously { network_header, ipv4, transport_header } or { network_header,
ipv6, transport_header }.

Change-Id: I74b1e0494a43eae9a9f91dca575d5419f3907168
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/77172
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-by: Hudson CI