build: enable -Wshadow

Note about shada.c:
- shada_read_next_item_start was intentionally shadowing `unpacked` and
  `i` because many of the macros (e.g. ADDITIONAL_KEY) implicitly
  depended on those variable names.
- Macros were changed to parameterize `unpacked` (but not `i`). Macros
  like CLEAR_GA_AND_ERROR_OUT do control-flow (goto), so any other
  approach is messy.

src/nvim/misc1.c

@@ -203,7 +203,7 @@ open_line (
       char_u  *ptr;
       char_u last_char;
 
-      pos_T old_cursor = curwin->w_cursor;
+      old_cursor = curwin->w_cursor;
       ptr = saved_line;
       if (flags & OPENLINE_DO_COM)
         lead_len = get_leader_len(ptr, NULL, FALSE, TRUE);