Fix crash when creating index from index

Fix crash when creating index from index

My patches yesterday to unify the DWARF index base classes had a bug
-- namely, I did the wholesale dynamic_cast-to-static_cast too hastily
and introduced a crash.  This can be seen by trying to add an index to
a file that has an index, or by running a test like gdb-index-cxx.exp
using the cc-with-debug-names.exp target board.

This patch fixes the crash by introducing a new virtual method and
removing some of the static casts.