gdb/ctf: don't create a buildsym_compunit when building partial symbols

I am trying to do some changes to buildsym_compunit, so I am auditing
the current uses.  Something seems odd with this use of
buildsym_compunit (that this patch removes).

A buildsym_compunit is normally used when building a compunit_symtab.
That is, when expanding a partial symtab into a full compunit symtab.
In ctfread.c, a buildsym_compunit is created in ctf_start_archive, which
is only used when creating partial symtabs.  At this moment, I don't
see how that's useful.  ctf_start_archive creates a new
buildsym_compunit and starts a subfile.  But that buildsym_compunit is
never used again.  It's just overriden in ctf_start_symtab, which means
we leak the old buildsym_compunit, I suppose.

Remove ctf_start_archive completely.  Add an assert in
ctf_start_symtab to verify that we are not overwriting an existing
buildsym_compunit (meaning we'd leak the existing one).  This assert
triggers without the other part of the fix.  When doing:

  $ ./gdb --data-directory=data-directory /tmp/babeltrace-ctf/src/lib/.libs/libbabeltrace2.so.0.0.0
  ...
  (gdb) maintenance expand-symtabs
  /home/simark/src/binutils-gdb/gdb/ctfread.c:1255: internal-error: ctf_start_symtab: Assertion `!ccp->builder' failed.

Change-Id: I666d146454a019f08e7305f3a1c4a974d27b4592

gdb/ctfread.c

@@ -1531,21 +1531,6 @@ ctf_psymtab_var_cb (const char *name, ctf_id_t id, void *arg)
   return 0;
 }
 
-/* Start a subfile for CTF. FNAME is the name of the archive.  */
-
-static void
-ctf_start_archive (struct ctf_context *ccx, struct objfile *of,
-		   const char *fname)
-{
-  if (ccx->builder == nullptr)
-    {
-      ccx->builder = new buildsym_compunit (of,
-		      of->original_name, nullptr, language_c, 0);
-      ccx->builder->record_debugformat ("ctf");
-    }
-  ccx->builder->start_subfile (fname);
-}
-
 /* Setup partial_symtab's describing each source file for which
    debugging information is available.  */
 
@@ -1567,10 +1552,7 @@ scan_partial_symbols (ctf_dict_t *cfp, psymtab_storage *partial_symtabs,
 
   struct ctf_context *ccx = &pst->context;
   if (isparent == false)
-    {
-      ctf_start_archive (ccx, of, fname);
-      ccx->pst = pst;
-    }
+    ccx->pst = pst;
 
   if (ctf_type_iter (cfp, ctf_psymtab_type_cb, ccx) == CTF_ERR)
     complaint (_("ctf_type_iter scan_partial_symbols failed - %s"),