interpret: ensure that calls via no-unwind ABIs do not unwind

[RalfJung]

According to our ABI docs, programs like this are okay:

extern "C-unwind" fn does_not_unwind_but_could() {}

fn main() {
    let f: extern "C-unwind" fn() = does_not_unwind_but_could;
    let f: extern "C" fn() = unsafe { std::mem::transmute(f) };
    f();
}

So let's add a test for that.

And also, let's adjust the checks in Miri's shims accordingly (see src/tools/miri/src/shims/sig.rs). We used to reject calls to functions that might unwind with a signature that does not allow unwinding, even if no unwinding occurred. I don't think we have an actual example of a potentially-unwinding shim with an ABI that has a compatible ABI that does not allow unwinding ("C-unwind" and "C"), so we can't add a test for this.

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This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

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