macOS: linker crash "unsupported reference (alias of) to class-ref" (ClassRefToGotConverter) linking FirebaseCore + FirebaseFirestore frameworks on Xcode 27

### Environment

- Firebase C++ SDK: 13.12.0 (its macOS `frameworks/darwin` distribution wraps the underlying Firebase Apple SDK; "macOS SDKs are now built using Xcode 26.2" per the C++ SDK release notes starting around 13.8.0)
- Xcode: 27.0 (27A266a)
- clang: Apple clang version 21.0.0 (clang-2100.3.34.2)
- ld: version 27037.1 (new/default linker)
- macOS: 26.6.2 (build 25G83)
- Architecture: arm64 (Apple Silicon)
- `MACOSX_DEPLOYMENT_TARGET`: 15.7
- Frameworks linked: `firebase.framework`, `firebase_firestore.framework`, `firebase_auth.framework`, `firebase_analytics.framework`, `firebase_functions.framework`, `firebase_messaging.framework`, `firebase_remote_config.framework` (the macOS desktop frameworks bundled by the Firebase C++ SDK, which wrap the Apple SDK)

### What happens

Linking a macOS desktop app against `firebase.framework` + `firebase_firestore.framework` fails with a linker crash (not a normal "undefined symbol" error — the linker itself asserts and aborts):

```
ld: unsupported reference (alias of) to class-ref from '_f_b_OBJC_CLASSLIST_REFERENCES_$_' (firebase_firestore.framework/firebase_firestore[arm64][1145](9c7b11ffbad0c92d0adcf47d138d4f4c_error_apple.mm.o))
ld: unsupported reference (alias of) to class-ref from '_OBJC_CLASSLIST_REFERENCES_$_' (firebase.framework/firebase[arm64][181](2821dc7f0f996caf95294ccbe3bb0f98_locale_apple.mm.o))
0 0x10222f670 __assert_rtn + 252
1 0x1021813f0 ___ZN2ld4pass22ClassRefToGotConverter9transformEv_block_invoke_2 + 0
2 0x1825254c8 _dispatch_client_callout2 + 16
3 0x1825201a4 _dispatch_apply_invoke3 + 336
4 0x1825254b0 _dispatch_client_callout + 16
5 0x18250e630 _dispatch_once_callout + 32
6 0x18251f2f4 _dispatch_apply_invoke + 248
7 0x1825254b0 _dispatch_client_callout + 16
8 0x182542d6c _dispatch_channel_invoke.cold.9 + 28
9 0x18251dadc _dispatch_root_queue_drain + 708
10 0x18251e120 _dispatch_worker_thread2 + 184
11 0x1826c3e84 _pthread_wqthread + 232
ld: Assertion failed: (0 && "unsupported references to class-ref"), function changeClassRefUseToGotUse, file ObjC.cpp, line 1151.
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
```

It appears an Objective-C class reference is aliased across **two separate framework binaries** — one class-ref inside `firebase_firestore.framework`'s `error_apple.mm.o` and one inside `firebase.framework`'s `locale_apple.mm.o` — and the linker's new `ClassRefToGotConverter` pass hard-asserts instead of handling it.

### What we've tried

1. **`-ld_classic`** — no longer has any effect on Xcode 27: `ld: warning: -ld_classic is no longer supported and will be ignored`. Same crash occurs anyway.
2. **No special linker flags at all** — same crash. This confirms it's not specific to `-ld_classic`; the default linker crashes on its own.
3. **`-Xlinker -dead_strip -Xlinker -allow_dead_duplicates`** (suggested as a workaround for a related issue in #15284) — this does avoid the crash, but as a side effect the linker silently discards unrelated, legitimate symbols from other static libraries linked into the same binary (we saw `Undefined symbol` errors reappear afterward for FreeType, ICU, and other unrelated libraries), so it's not a safe workaround.

### Expected behavior

The prebuilt macOS frameworks should link without crashing the linker, ideally without requiring `-allow_dead_duplicates` (which has known correctness risks per Apple's own linker docs).

### Related issue

This looks closely related to #15284 (FirebaseAnalytics + `-ld_classic` on iOS), but reproduces here on macOS desktop with the default (non-classic) linker, so it isn't limited to `-ld_classic` usage.

### Question for the Firebase team

Is this a duplicate/aliased symbol artifact from how `firebase.framework` and `firebase_firestore.framework` are packaged for macOS (e.g., the same translation unit/class getting folded into an alias across two separate framework binaries)? Has this been reproduced internally on Xcode 27, and is there a recommended workaround in the meantime (e.g. pinning to an older SDK release built with an earlier Xcode)?

Commencez par reproduire l'édition de liens avec firebase.framework et firebase_firestore.framework dans l'environnement Xcode 27 indiqué, puis examinez l'échec de ClassRefToGotConverter impliquant error_apple.mm.o et locale_apple.mm.o. Comparez les flags de l'éditeur de liens et les versions du SDK testés décrits dans l'issue. La tâche est terminée lorsqu'un fix sûr ou un workaround documenté est identifié pour éditer les liens des frameworks macOS précompilés sans le crash signalé ni symboles manquants sans rapport.

Rédigé par le modèle d'indexation à partir du texte de l'issue.