gdb/dwarf: clean up some cutu_reader::is_dummy() calls

gdb/dwarf: clean up some cutu_reader::is_dummy() calls

This patch tries to standardize the places where we check if units are
dummy.  When checking if a unit is dummy, it is not necessary to check
for some other conditions.

 - cutu_reader::is_dummy() is a superset of cutu_reader::cu() returning
   nullptr, so it's not necessary to check if the cu method return
   nullptr if also checking if the unit is dummy.
 - cutu_reader::is_dummy() is a superset of cutu_reader::top_level_die()
   returning nullptr, so same deal.

Remove some spots that check for these conditions in addition to
cutu_reader::is_dummy().

In addition, also remove the checks for:

    !new_reader->top_level_die ()->has_children

in cooked_indexer::ensure_cu_exists.  IMO, it is not useful to special
case the units having a single DIE.  Especially in this function, which
deals with importing things from another unit, a unit with a single DIE
would be an edge case that should not happen with good debug info.  I
think it's preferable to have simpler code.

Change-Id: I4529d7b3a0bd2891a60f41671de8cfd3114adb4a
Approved-By: Tom Tromey <tom@tromey.com>