declare C and C-unwind as mutually ABI-compatible

[RalfJung]

We left this conservative in #115476. That means the following code is currently UB:

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();
}

I think this code should be allowed. Zulip discussion also led to the conclusion that this should be fine in LLVM -- the nounwind attribute does not affect the ABI. This is widely relied upon in the C ecosystem when doing calls between C and C++.
Cc @nikic to confirm.

I am not sure why #115476 left this conservative. There was discussion about explicitly listing the ABIs rather than saying this holds universally for all *-unwind that we may add in the future, but there was no discussion I could find about allowing a mismatch both ways.

Cc @rust-lang/opsem @rust-lang/lang

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