If the constructor of the contained object is not run (e.g. because
assembling its arguments triggered an exception), the dtor must not
be run when destructing the cling::Value. Detect this case by imprinting
canary bytes into the contained object bytes: if they have changed,
run the dtor, if not assume that the constructor has failed.

This will cause false positives in those cases where the constructor
is not modifying the first object bytes: in these cases, the dtor
is not run even though the ctor is run. That is still better than
the other case (where the dtor crashes because no ctor was run).