Auto merge of #96959 - nbdd0121:unwind, r=Amanieu

Prevent unwinding when `-C panic=abort` is used regardless declared ABI

Ensures that Rust code will abort with `-C panic=abort` regardless ABI used.
```rust
extern "C-unwind" {
    fn may_unwind();
}

// Will be nounwind with `-C panic=abort`, despite `C-unwind` ABI.
pub unsafe extern "C-unwind" fn rust_item_that_can_unwind() {
    may_unwind();
}
```

Current behaviour is that unwind will propagate through. While the current behaviour won't cause unsoundness it is inconsistent with the text reading of [RFC2945](https://rust-lang.github.io/rfcs/2945-c-unwind-abi.html).

I tweaked `fn_can_unwind` instead of tweaking `AbortUnwindingCalls` because this approach would allow Rust (non-direct) callers to also see that this function is nounwind, so it can prevent excessive landing pads generation.

For more discussions: https://rust-lang.zulipchat.com/#narrow/stream/210922-project-ffi-unwind/topic/soundness.20in.20mixed.20panic.20mode.

cc `@alexcrichton,` `@BatmanAoD`
r? `@Amanieu`

`@rustbot` label: T-compiler T-lang F-c_unwind

compiler/rustc_middle/src/ty/layout.rs

@@ -2888,6 +2888,14 @@ pub fn fn_can_unwind<'tcx>(tcx: TyCtxt<'tcx>, fn_def_id: Option<DefId>, abi: Spe
             return false;
         }
 
+        // With `-C panic=abort`, all non-FFI functions are required to not unwind.
+        //
+        // Note that this is true regardless ABI specified on the function -- a `extern "C-unwind"`
+        // function defined in Rust is also required to abort.
+        if tcx.sess.panic_strategy() == PanicStrategy::Abort && !tcx.is_foreign_item(did) {
+            return false;
+        }
+
         // With -Z panic-in-drop=abort, drop_in_place never unwinds.
         //
         // This is not part of `codegen_fn_attrs` as it can differ between crates