[5.6] IRGen: Fix using weak linkage to avoid duplicate metadata by only applying this to PROTOCOL related data

[aschwaighofer]

rdar://86256970

[aschwaighofer]

@swift-ci Please test

[compnerd]

@aschwaighofer would you mind adding a bit more detail to the commit message? What is "this"?

[aschwaighofer]

@compnerd "this" is using weak hidden linkage for objective-c metadata.

This PR is a cherry-pick of #40486.

[compnerd]

Ah! Thanks; that would be great to have in the commit message. BTW, for cherry-picks, the -x option is very helpful for tracking the original commit.

[aschwaighofer]

We originally used internal linkage which causes duplicate metadata entries.

I then applied weak hidden linkage generally to all objective-c metadata (in #40342). This causes linkage issues with CATEGORY objective-c metadata. The linker applies some optimization to category metadata that does not expect to see weak symbols. So, this PR selectively applies weak hidden linkage only to PROTOCOL objective c metadata.

[compnerd]

Hmm, I assume that weak hidden translates to internal ODR? The category metadata needs to be visible across module boundaries, and the category DCE that ld64 does as an optimization over ObjC metadata will require it to be weak ODR so that it can iterate and drop the category. The change itself seems reasonable, its purely the commit message which I was hoping could be improved so that future readers can follow the change. The reference to the cherry-picked commit would also work - as I believe that the original commit had a bit more context in the commit message.

[aschwaighofer]

I have updated the commit message.

[aschwaighofer]

@swift-ci Please test