gdb: add all_objfiles_removed observer

The new_objfile observer is currently used to indicate both when a new
objfile is added to program space (when passed non-nullptr) and when all
objfiles of a program space were just removed (when passed nullptr).
I think this is confusing (and Andrew apparently thinks so too [1]).
Add a new "all_objfiles_removed" observer to remove the second role from
"new_objfile".

Some existing users of new_objfile do nothing if the passed objfile is
nullptr.  For them, we can simply drop the nullptr check.  For others,
add a new all_objfiles_removed callback, and refactor things a bit to
keep the existing behavior as much as possible.

Some callbacks relied on current_program_space, and following
the refactoring now use either objfile->pspace or the pspace passed to
all_objfiles_removed.  I think this should be relatively safe, and in
general a step in the right direction.

On the notify side, I found only one call site to change from
new_objfile to all_objfiles_removed, in clear_symtab_users.  It is not
entirely clear to me that this is entirely correct.  clear_symtab_users
appears to be called in spots that don't remove all objfiles
(functions finish_new_objfile, remove_symbol_file_command, reread_symbols,
do_module_cleanups).  But I think that this patch at least makes the
current code clearer.

[1] a0a031bce0

Change-Id: Icb648f72862e056267f30f44dd439bd4ec766f13
Approved-By: Tom Tromey <tom@tromey.com>

gdb/python/py-inferior.c

@@ -190,20 +190,22 @@ python_new_objfile (struct objfile *objfile)
   if (!gdb_python_initialized)
     return;
 
-  gdbpy_enter enter_py (objfile != NULL
-			? objfile->arch ()
-			: target_gdbarch ());
+  gdbpy_enter enter_py (objfile->arch ());
 
-  if (objfile == NULL)
-    {
-      if (emit_clear_objfiles_event (current_program_space) < 0)
-	gdbpy_print_stack ();
-    }
-  else
-    {
-      if (emit_new_objfile_event (objfile) < 0)
-	gdbpy_print_stack ();
-    }
+  if (emit_new_objfile_event (objfile) < 0)
+    gdbpy_print_stack ();
+}
+
+static void
+python_all_objfiles_removed (program_space *pspace)
+{
+  if (!gdb_python_initialized)
+    return;
+
+  gdbpy_enter enter_py (target_gdbarch ());
+
+  if (emit_clear_objfiles_event (pspace) < 0)
+    gdbpy_print_stack ();
 }
 
 /* Emit a Python event when an objfile is about to be removed.  */
@@ -1020,6 +1022,8 @@ gdbpy_initialize_inferior (void)
   gdb::observers::new_objfile.attach
     (python_new_objfile, "py-inferior",
      { &auto_load_new_objfile_observer_token });
+  gdb::observers::all_objfiles_removed.attach (python_all_objfiles_removed,
+					       "py-inferior");
   gdb::observers::free_objfile.attach (python_free_objfile, "py-inferior");
   gdb::observers::inferior_added.attach (python_new_inferior, "py-inferior");
   gdb::observers::inferior_removed.attach (python_inferior_deleted,