Fix oddity in write_gdbindex

My recent patch to unify CUs and TUs introduced an oddity in
write_gdbindex.  Here, we pass 'i' to recursively_write_psymbols, but
we must instead pass 'counter', to handle the situation where a TU is
mixed in with the CUs.

I am not sure a test case for this is possible.  I think it can only
happen when using DWARF 5, where a TU appears in .debug_info.
However, this situation is already not handled correctly by
.gdb_index.  I filed a bug about this.

gdb/dwarf2/index-write.c

@@ -1441,7 +1441,7 @@ write_gdbindex (dwarf2_per_objfile *per_objfile, FILE *out_file,
 	{
 	  if (psymtab->user == NULL)
 	    recursively_write_psymbols (objfile, psymtab, &symtab,
-					psyms_seen, i);
+					psyms_seen, counter);
 
 	  const auto insertpair = cu_index_htab.emplace (psymtab, counter);
 	  gdb_assert (insertpair.second);