refactor: fix clang-tidy warnings

Enable and fix bugprone-misplaced-widening-cast warning.

Fix some modernize-macro-to-enum and readability-else-after-return
warnings, but don't enable them. While the warnings can be useful, they
are in general too noisy to enable.

src/nvim/ex_cmds.c

@@ -2137,9 +2137,8 @@ static int check_readonly(int *forceit, buf_T *buf)
         // Set forceit, to force the writing of a readonly file
         *forceit = true;
         return false;
-      } else {
-        return true;
       }
+      return true;
     } else if (buf->b_p_ro) {
       emsg(_(e_readonly));
     } else {