cpp-common/bt2: make `{OptionalBorrowedObject,SharedObject}::operator bool` explicit

cpp-common/bt2: make `{OptionalBorrowedObject,SharedObject}::operator bool` explicit

This code doesn't really make sense (what I really wanted to do was to
compare the underlying lib pointers), but I made this mistake on more
than one occasion:

    if (someOptionalBorrowedObject == someSharedObject)

What happens is: since `operator bool` for both types is implicit, they
get implicitly converted to bool.  As a result, the condition checks if
both objects contain an object or don't contain an object.  I consider
this a pitfall, as it compiles but it is likely never what we'll want to
do.

Make `operator bool` for both types explicit to avoid that.  This is
what is done in the STL for `shared_ptr` [1] and `optional` [2].

Note that even if `operator bool` is explicit, it remains implicitly
convertible to bool in certain contexts [3], such as an if condition.

[1] https://en.cppreference.com/w/cpp/memory/shared_ptr/operator_bool
[2] https://en.cppreference.com/w/cpp/utility/optional/operator_bool
[3] https://stackoverflow.com/questions/39995573/when-can-i-use-explicit-operator-bool-without-a-cast

Change-Id: I2b2377323ca620162442cb7f0305763c034d13d8
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12043
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>