[llvm] [MachO] Preserve weak aliases and fix alias n_desc flags (PR #212001)

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AsakuraMizu wrote:

> hi all, I'll have limited availability in coming days and may not be able to update promptly. Is anyone able to take over? Thanks

Hi @syhhyl, I picked this up and submitted #219534. It retains the `AsmPrinter` approach from this PR and revises the `MachObjectWriter` change based on the subsequent discussion. I intend it to supersede this PR.

Thanks for the work here!

https://github.com/llvm/llvm-project/pull/212001