Work around issue with clang implicit instantiation in C++ mode.

GaudiHandleBase has a virtual destructor which is declared with `=default'.
However. in C++20 mode, clang will then try to instiantiate all the virtual
functions for classes derived from this whenever the declarations
for those classes are seen.  The upshot is that then using Gaudi handles
with forward-declared classes doesn't work (even if the handle is only
declared in the header but not used in any way).

It turns out that we can avoid this behavior by changing the declaration
of the base class destructor to say `{}' instead of `=default'.
(This is perhaps a clang bug, but the standard is not very clear
on exactly when virtual members of the templated class are required
to be or not to be implicitly instantiated.)

See also ATLINFR-5113.