Ignore .debug_types when reading .debug_aranges

I noticed that the fission-reread.exp test case can cause a complaint
when run with --target_board=cc-with-debug-names:

warning: Section .debug_aranges in [...]/fission-reread has duplicate debug_info_offset 0x0, ignoring .debug_aranges.

The bug here is that this executable has both .debug_info and
.debug_types, and both have a CU at offset 0x0.  This triggers the
duplicate warning.

Because .debug_types doesn't provide any address ranges, these CUs can
be ignored.  That is, this bug turns out to be another regression from
the info/types merger patch.

This patch fixes the problem by having this loop igore type units.
fission-reread.exp is updated to test for the bug.

gdb/dwarf2/read.c

@@ -2536,6 +2536,12 @@ create_addrmap_from_aranges (dwarf2_per_objfile *per_objfile,
     debug_info_offset_to_per_cu;
   for (const auto &per_cu : per_bfd->all_comp_units)
     {
+      /* A TU will not need aranges, and skipping them here is an easy
+	 way of ignoring .debug_types -- and possibly seeing a
+	 duplicate section offset -- entirely.  */
+      if (per_cu->is_debug_types)
+	continue;
+
       const auto insertpair
 	= debug_info_offset_to_per_cu.emplace (per_cu->sect_off,
 					       per_cu.get ());

gdb/testsuite/gdb.dwarf2/fission-reread.exp

@@ -47,7 +47,8 @@ if [build_executable_and_dwo_files "$testfile.exp" "${binfile}" $options \
     return -1
 }
 
-clean_restart $binfile
+clean_restart
+gdb_load_no_complaints $binfile
 
 gdb_test "break -q main" "Breakpoint.*at.*"