gdb/dwarf: remove all_{comp,type}_units views

gdb/dwarf: remove all_{comp,type}_units views

In DWARF 5, type units appear in the .debug_info section, interleaved
with comp units, and the order in all_units reflects that.  The
all_comp_units and all_type_units views are wrong in that case
(all_comp_units contains some type units, and vice-versa).

It would be possible to manually sort all_units to ensure that type
units follow comp units, but this series takes the approach of sorting
the units by section and section offset.

Remove those views, and replace their uses with num_comp_units and
num_type_units.  It appears that the views were only used to know the
number of each kind.

The finalize_all_units function is now empty, but I am keeping it
because a subsequent patch adds a call to std::sort in there to sort the
all_units vector.

Change-Id: I42a65b6f1b6192957b55cea0e2eaff097e13a33b
Approved-By: Tom Tromey <tom@tromey.com>