gdb/dwarf: sort units when writing index

gdb/dwarf: sort units when writing index

The order of all_units can't be relied on when writing the CU and TU
lists to .gdb_index or .debug_names.

Both the .gdb_index and .debug_names writers expect that all_units
contains comp units followed by type units.  As of this commit, when
reading a DWARF 5 .debug_info, the all_units vector is ordered based on
the order the units appear in .debug_info, where type units can be
interleaved with comp units.

It probably worked fine with DWARF 4, where type units were in a section
of their own (.debug_types).  They were read after comp units, and
therefore after them in the all_units vector.

Change the writers to use a common function that splits the units in two
lists (comp units and type units).  Sort both lists by section offset.
This is more than required, but it should help produce a stable and
predictable output.

Change-Id: I5a22e2e354145e3d6b5b2822dc2a3af2f9d6bb76
Approved-By: Tom Tromey <tom@tromey.com>