Rollup merge of #143060 - xizheyin:simplify-diag, r=oli-obk

Only args in main diag are saved and restored without removing the newly added ones

cc rust-lang/rust#142724

Here's a more simplified approach, since we'll be storing and restoring the main diagnostic's arg, removing args newly added isn't needed in the derive subdiagnostic implementation. `remove_arg` is helpful only for manual implementation of subdiagnostic.

r? ``@oli-obk``

Author: GuillaumeGomez