*: Use __attribute__((fallthrough)) where comments aren't supported

Although GCC now detects possibly unintentional fall through, there
rules around which the comments are detected are rather strict.  In
cases where a comment isn't detected, upstream [recommends] using their
fallthrough attribute.

[recommends]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77817#c11

src/nvim/eval/typval.c

@@ -1869,7 +1869,7 @@ void tv_free(typval_T *tv)
       }
       case VAR_FUNC: {
         func_unref(tv->vval.v_string);
-        // FALLTHROUGH
+        FALLTHROUGH;
       }
       case VAR_STRING: {
         xfree(tv->vval.v_string);