Auto merge of #30778 - fhahn:issue-21195-expect-help, r=nikomatsakis

Auto merge of #30778 - fhahn:issue-21195-expect-help, r=nikomatsakis

This is a PR for #21195. It changes the way unspecified `help` and `ǹote` messages are handled in compile-fail tests as suggested by @oli-obk in the issue: if there are some `note` or `help` annotations, there must be annotations for all `help` or `note` messages of this test. Maybe it makes also sense to add an option to specify that the this test should fail if there are unspecified `help` or `note` messages.

With this change, the following tests fail:

    [compile-fail] compile-fail/changing-crates.rs
    [compile-fail] compile-fail/default_ty_param_conflict_cross_crate.rs
    [compile-fail] compile-fail/lifetime-inference-give-expl-lifetime-param.rs
    [compile-fail] compile-fail/privacy1.rs
    [compile-fail] compile-fail/svh-change-lit.rs
    [compile-fail] compile-fail/svh-change-significant-cfg.rs
    [compile-fail] compile-fail/svh-change-trait-bound.rs
    [compile-fail] compile-fail/svh-change-type-arg.rs
    [compile-fail] compile-fail/svh-change-type-ret.rs
    [compile-fail] compile-fail/svh-change-type-static.rs
    [compile-fail] compile-fail/svh-use-trait.rs

I'll add the missing annotations if we decide to accept this change.