Enable the new DWARF indexer

Enable the new DWARF indexer

This patch finally enables the new indexer.  It is left until this
point in the series to avoid any regressions; in particular, it has to
come after the changes to the DWARF index writer to avoid this
problem.

However, if you experiment with the series, this patch can be moved
anywhere from the patch to wire in the new reader to this point.
Moving this patch around is how I got separate numbers for the
parallelization and background finalization patches.

In the ongoing performance example, this reduces the time from the
baseline of 1.598869 to 0.903534.