refactor: reorganize_definitions: use the fallback/C ABI instead of Rust

Both sites now use `Abi::FALLBACK` (= `Abi::C { unwind: false }`).

`Regression test: `test_reorganize_implicit_extern`
(`tests/snapshots/reorganize_implicit_extern.rs`); on pre-fix code its
implicit-`extern` declaration fails to merge with the `extern "C"` duplicate,
producing an `extern "Rust"` block and a spurious `compute_1` rename.

`reorganize_definitions.rs:843-847` and `:1779-1782` both do:

```rust
let abi = m.abi.and_then(|abi| abi::lookup(&abi.symbol.as_str())).unwrap_or(Abi::Rust);
```

Rust's default ABI for `extern {}` blocks is `"C"` (rustc's own lowering uses
`Abi::FALLBACK`, which is `Abi::C`). Consequences:

- An implicit `extern { fn foo(); }` block never dedups against an
  `extern "C" { fn foo(); }` declaration — the ABI-mismatch `continue` at
  `:2144` in `find_foreign_item` skips it.
- Worse, its items are re-emitted by `into_items` via
  `mk().extern_(Abi::Rust)` (`:1990`), silently changing the declared ABI of
  foreign functions.

Latent only because transpiled code usually writes `extern "C"` explicitly.
Fix: fall back to `Abi::C { unwind: false }` (or `Abi::FALLBACK`).

Author: ahomescu

c2rust-refactor/src/transform/reorganize_definitions.rs

@@ -881,10 +881,12 @@ impl<'a, 'tcx> Reorganizer<'a, 'tcx> {
                         .into_iter()
                         .filter_map(|mut item| {
                             if let ItemKind::ForeignMod(m) = &mut item.kind {
+                                // `extern` without an explicit ABI string defaults
+                                // to "C", same as rustc's lowering.
                                 let abi = m
                                     .abi
                                     .and_then(|abi| abi::lookup(&abi.symbol.as_str()))
-                                    .unwrap_or(Abi::Rust);
+                                    .unwrap_or(Abi::FALLBACK);
                                 m.items.retain(|item| {
                                     match declarations.find_foreign_item(item, abi) {
                                         ContainsDecl::NotContained => true,
@@ -1847,10 +1849,12 @@ impl<'a, 'tcx> HeaderDeclarations<'a, 'tcx> {
             // defined in ident_map after processing the whole list of items.
             ItemKind::ForeignMod(f) => {
                 for item in f.items.iter() {
+                    // `extern` without an explicit ABI string defaults to "C",
+                    // same as rustc's lowering.
                     let abi = f
                         .abi
                         .and_then(|abi| abi::lookup(&abi.symbol.as_str()))
-                        .unwrap_or(Abi::Rust);
+                        .unwrap_or(Abi::FALLBACK);
                     self.insert_foreign_item(item.clone(), abi, parent_header.clone());
                 }
                 true

c2rust-refactor/tests/snapshots/snapshots__refactor-reorganize_definitions-reorganize_implicit_extern.rs.snap

@@ -11,7 +11,7 @@ expression: c2rust-refactor reorganize_definitions --rewrite-mode alongside -- t
 pub mod first {
 
     // =============== BEGIN first_h ================
-    extern "Rust" {
+    extern "C" {
 
         pub fn compute(x: i32) -> i32;
     }
@@ -23,15 +23,10 @@ pub mod first {
 
 pub mod second {
 
-    // =============== BEGIN second_h ================
-    extern "C" {
-
-        #[link_name = "compute"]
-        pub fn compute_1(x: i32) -> i32;
-    }
+    use crate::first::compute;
 
     pub fn call_second(x: i32) -> i32 {
-        unsafe { crate::second::compute_1(x) }
+        unsafe { crate::first::compute(x) }
     }
 }