gdb: clear internalvar on destruction

The data associated to an internalvar is destroyed when changing the
kind of the internalvar, but not when it is destroyed.  Fix that by
calling clear_internalvar in ~internalvar.

A move constructor becomes needed to avoid freeing things multiple times
when internalvars are moved (and if we forget it, clang helpfully gives
us a -Wdeprecated-copy-with-user-provided-dtor warning).

Change-Id: I427718569208fd955ea25e94d341dde356725033
Reviewed-By: Guinevere Larsen <guinevere@redhat.com>