[llvm] [MachO] Preserve weak linkage for aliases (PR #198148)

[via llvm-commits]

syhhyl wrote:

I performed the requested end-to-end testing with rustc at eff8269f797067c30555e77f160ec84c0ed15cd9 on Apple arm64.

I applied this LLVM change to the LLVM fork used by rustc, temporarily disabled rustc's Apple-specific restriction on static EII defaults for testing, and ran the following stage 1 tests:

- tests/ui/eii/static/default.rs
- tests/ui/eii/static/default_cross_crate.rs
- tests/ui/eii/static/default_cross_crate_explicit.rs

All three tests passed. In particular, the cross-crate test successfully compiled, linked, and ran, covering the original failure mode where the weak alias was not externally linkable.

https://github.com/llvm/llvm-project/pull/198148