message: Fix warnings from better precision of `dead_code` lint
  The lint now ignores derived `Clone` and `Debug` implementations, as of
  PR rust-lang/rust#85200, which landed a couple of days ago.
  I sprinkled `#[allow(dead_code)]` in a few places; the fields are not
  expected to be read since they are just part of a very specific test.
diff: @@ -656,6 +656,7 @@ Caused by:
  );
  #[derive(Debug, Deserialize)]
  + #[allow(dead_code)]
  struct S {
  f1: i64,
  f2: String,
  @@ -1155,6 +1156,7 @@ fn table_merge_failure() {
  );
  #[derive(Debug, Deserialize)]
  + #[allow(dead_code)]
  struct Table {
  key: StringList,
  }
  