[gdb/symtab] Error out for .debug_types section in dwz file

There are two methods to factor out type information in a dwarf4 executable:
- use -fdebug-info-types to generate type units in a .debug_types section, and
- use dwz to create partial units.

The dwz method has an extra benefit: it also allows to factor out information
between executables into a newly created .dwz file, pointed to by a
.gnu_debugaltlink section.

There is nothing prohibiting a .gnu_debugaltlink file to contain a
.debug_types section.

It's just not generated by dwz or any other tool atm, and consequently gdb has
no support for it.  Enhancement PR symtab/30838 is open about the lack of
support.

Make the current situation explicit by emitting a dwarf error:
...
(gdb) file struct-with-sig-2^M
Reading symbols from struct-with-sig-2...^M
Dwarf Error: .debug_types section not supported in dwz file^M
...
and add an assert in write_gdbindex:
...
+      /* See enhancement PR symtab/30838.  */
+      gdb_assert (!(per_cu->is_dwz && per_cu->is_debug_types));
...
to clarify why we can use:
...
      data_buf &cu_list = (per_cu->is_debug_types
                           ? types_cu_list
                           : per_cu->is_dwz ? dwz_cu_list : objfile_cu_list);
...

The test-case is a modified copy from gdb.dwarf2/struct-with-sig.exp, so it
keeps the copyright years range.

Tested on x86_64-linux.

Tested-By: Guinevere Larsen <blarsen@redhat.com>

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30838