Wire in the new DWARF indexer

Wire in the new DWARF indexer

This wires the new DWARF indexer into the existing reader code.  That
is, this patch makes the modification necessary to enable the new
indexer.  It is not actually enabled by this patch -- that will be
done later.

I did a bit of performance testing for this patch and a few others.  I
copied my built gdb to /tmp, so that each test would be done on the
same executable.  Then, each time, I did:

    $ ./gdb -nx
    (gdb) maint time 1
    (gdb) file /tmp/gdb

This patch is the baseline and on one machine came in at 1.598869 wall
time.