Dead code pass no longer considers enum used in `pub extern "C" fn` as used

A repository that I maintain recently received a bug report about our proc macro generating a warning after they upgraded to 1.79.0.

I haven't bisected for the exact commit that introduces the warning, but I have been able to reproduce it in 1.79.0.

Minimal Reproduction

fn main() {}

mod private_mod {
    #[repr(C)]
    pub enum MyEnum {
        Variant(u32)
    }
    
    #[no_mangle]
    pub extern "C" fn some_function() -> MyEnum {
        MyEnum::Variant(5)
    }
}

   Compiling playground v0.0.1 (/playground)
warning: field `0` is never read
 --> src/main.rs:6:17
  |
6 |         Variant(u32)
  |         ------- ^^^
  |         |
  |         field in this variant
  |
  = note: `#[warn(dead_code)]` on by default
help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
  |
6 |         Variant(())
  |                 ~~

warning: `playground` (bin "playground") generated 1 warning
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.85s
     Running `target/debug/playground`

In the playground -> https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=3f5c81c0749130a6e46fbea14cbb055f

Potential Candidates

This comment links to some rustc commits that may have introduced the issue https://github.com/chinedufn/swift-bridge/pull/278#issuecomment-2179226690

As mentioned, I haven't taken the time to bisect since I know that it's possible that a core maintainer already has a good sense of when/how this regression could have been introduced.

Start by running the minimal Rust reproduction and comparing the dead-code behavior across versions, then inspect the rustc commits linked in the referenced swift-bridge comment. Trace the dead-code pass for a repr(C) enum returned by a pub extern "C" function; done means a regression test covers this case and no spurious field warning is emitted.