Change naming convention for enum class enumerators

Change naming convention for enum class enumerators

Change the naming convention for enum class enumerators to not use
`ALL_CAPS`, as suggested by the C++ Core Guidelines [1].  Instead, use
camel case starting with a capital letter.

Update `CONTRIBUTING.adoc` to reflect this change.

gcc emits warnings like this:

    /home/smarchi/src/babeltrace/src/cpp-common/bt2/trace-ir.hpp:880:9: error: declaration of 'bt2::CommonEventClass<LibObjT>::LogLevel::Error' shadows a global declaration [-Werror=shadow]
      880 |         Error = BT_EVENT_CLASS_LOG_LEVEL_ERROR,
          |         ^~~~~
    In file included from /home/smarchi/src/babeltrace/src/cpp-common/bt2/clock-class.hpp:20,
                     from /home/smarchi/src/babeltrace/src/clock-correlation-validator/clock-correlation-validator.cpp:7:
    /home/smarchi/src/babeltrace/src/cpp-common/bt2/exc.hpp:14:7: note: shadowed declaration is here
       14 | using Error = bt2c::Error;
          |       ^~~~~

Whether or not this is a compiler bug is not clear [2], but in our case
we know it's fine.  Add and use `BT_DIAG_IGNORE_SHADOW` in some places
to silence these warnings.

[1] https://cpp-core-guidelines-docs.vercel.app/enum#Renum-caps
[2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55776

Change-Id: I59bb7f3c8572106c325596bc80a4d70714d86dee
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12212
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>