Unify the DWARF index holders

The dwarf2_per_bfd object has a separate field for each possible kind
of index.  Until an earlier patch in this series, two of these were
even derived from a common base class, but still had separate slots.
This patch unifies all the index fields using the common base class
that was introduced earlier in this series.  This makes it more
obvious that only a single index can be active at a time, and also
removes some code from dwarf2_initialize_objfile.