gdb: pass frames as const frame_info_ptr &

We currently pass frames to function by value, as `frame_info_ptr`.
This is somewhat expensive:

 - the size of `frame_info_ptr` is 64 bytes, which is a bit big to pass
   by value
 - the constructors and destructor link/unlink the object in the global
   `frame_info_ptr::frame_list` list.  This is an `intrusive_list`, so
   it's not so bad: it's just assigning a few points, there's no memory
   allocation as if it was `std::list`, but still it's useless to do
   that over and over.

As suggested by Tom Tromey, change many function signatures to accept
`const frame_info_ptr &` instead of `frame_info_ptr`.

Some functions reassign their `frame_info_ptr` parameter, like:

  void
  the_func (frame_info_ptr frame)
  {
    for (; frame != nullptr; frame = get_prev_frame (frame))
      {
        ...
      }
  }

I wondered what to do about them, do I leave them as-is or change them
(and need to introduce a separate local variable that can be
re-assigned).  I opted for the later for consistency.  It might not be
clear why some functions take `const frame_info_ptr &` while others take
`frame_info_ptr`.  Also, if a function took a `frame_info_ptr` because
it did re-assign its parameter, I doubt that we would think to change it
to `const frame_info_ptr &` should the implementation change such that
it doesn't need to take `frame_info_ptr` anymore.  It seems better to
have a simple rule and apply it everywhere.

Change-Id: I59d10addef687d157f82ccf4d54f5dde9a963fd0
Approved-By: Andrew Burgess <aburgess@redhat.com>

gdb/python/python-internal.h

@@ -384,7 +384,7 @@ extern enum ext_lang_rc gdbpy_apply_val_pretty_printer
    const struct language_defn *language);
 extern enum ext_lang_bt_status gdbpy_apply_frame_filter
   (const struct extension_language_defn *,
-   frame_info_ptr frame, frame_filter_flags flags,
+   const frame_info_ptr &frame, frame_filter_flags flags,
    enum ext_lang_frame_args args_type,
    struct ui_out *out, int frame_low, int frame_high);
 extern void gdbpy_preserve_values (const struct extension_language_defn *,
@@ -444,7 +444,7 @@ PyObject *block_to_block_object (const struct block *block,
 				 struct objfile *objfile);
 PyObject *value_to_value_object (struct value *v);
 PyObject *type_to_type_object (struct type *);
-PyObject *frame_info_to_frame_object (frame_info_ptr frame);
+PyObject *frame_info_to_frame_object (const frame_info_ptr &frame);
 PyObject *symtab_to_linetable_object (PyObject *symtab);
 gdbpy_ref<> pspace_to_pspace_object (struct program_space *);
 PyObject *pspy_get_printers (PyObject *, void *);