bt2::internal::BorrowedObj: add static assertions for generic copy ops.

bt2::internal::BorrowedObj: add static assertions for generic copy ops.

This patch adds static assertions to `borrowed-obj.hpp` so as to make
compiler errors more readable when assigning a const wrapper to a
non-const wrapper, for example:

    bt2::Value v = bt2::ConstMapValue {somePtr};

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ib23d91c6660a478e0ad663abbd141b0a90eaa92d
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8091
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10802
Tested-by: jenkins <jenkins@lttng.org>