Clarifying the behavior of `#[rustc_nounwind]` at trait methods

Currently, #[rustc_nounwind] is only applied to functions that are explicitly annotated; it's neither required nor applied by default to all implementations. I have no objection to this, but I think we need to clarify what the expected behavior should be.

An alternative is:

If you apply #[rustc_nounwind] on a trait method, it should be fine to assume that all calls of this trait method don't unwind, right? If any of the implementations doesn't use #[rustc_nounwind], that should probably be reported as error. (from https://github.com/rust-lang/rust/pull/137669#discussion_r1973326456 @bjorn3)

pub trait Trait {
    #[rustc_nounwind]
    fn m(&self, _: (i32, i32, i32));
}

impl Trait for () {
    fn m(&self, _: (i32, i32, i32)) {
        panic!();
    }
}

Based on the current behavior, there are some miscompile.
In the case of https://rust.godbolt.org/z/aTWsEMxWq, all invokes should be unwinding.

From #137669.

Start by reading the discussion in #137669 and the linked Compiler Explorer example to understand the reported miscompile around #[rustc_nounwind] on trait methods. Trace the current behavior for the trait declaration and its implementations, then establish the intended rule and add coverage showing that calls and implementations follow it.