Fixing order of --warn_error

[mtzguido]

The --warn_error option is accumulated by successive uses, with latter uses overriding the former, hence --warn_error @1..100 --warn_error -50 will ignore error 50, but make 1..49 and 51..100 fatal.

However, combinations within a single --warn_error option use have the order reversed, the equivalent to the above is --warn_error [email protected].

This PR changes that, so the order within a single invocation is left-to-right as for separate invocations, and hence --warn_error A --warn_error B is the same as --warn_error AB. Earlier settings are overridden by latter ones (i.e. to the right, or more deeply nested within #push-options).

Motivated by #2950

[mtzguido]

None of our projects seem to rely on the current behavior (I got an everest green). Pinging @tahina-pro and @msprotz just in case they have any thoughts. (FWIW this is also what OCaml does).