[IRGen] Use PrivateLinkage for @__Swift_AST global

[kateinoigakukun]

.swift_ast section is now recognized as a "metadata" section by LLVM's object emission. On WebAssembly, metadata symbols must not appear in the object file symbol table. Therefore, change the linkage of the @__Swift_AST global from InternalLinkage to PrivateLinkage to avoid exposing the symbol.

swiftlang/llvm-project@555eaef

[kateinoigakukun]

@swift-ci smoke test

[aschwaighofer]

Seems reasonable to me.

[aschwaighofer]

Does the corresponding LLVM patch also existing on the branch that main currently builds with?

[kateinoigakukun]

The commit is not included in the current stable llvm branch swift's main uses but it's included in the new rebranch. This is a fix to unblock the ongoing test failures on rebranch

[aschwaighofer]

Should this change be merged to rebranch instead? Is there the risk that the symbol will be stripped (in main branch) without the matching llvm commit on the stable/.... branch?

I am inferring that possibility from the comment on the llvm PR:

This change ensures that this section is not marked as loadable, which makes it possible for it to be stripped.

[kateinoigakukun]

I think it's safe to apply the LLVM change and this change independently, because a private loadable global var should remain in the object file. Correct me @hyp if my understanding is incorrect.
My intention to merge this to main instead rebranch is to minimize dfff for the rebranch merge

[aschwaighofer]

Aah, I misread/misunderstood what the LLVM change does: it allows for stripping the symbol, not as I initially read it, prevent it.

[AnthonyLatsis]

@swift-ci please smoke test Linux

[AnthonyLatsis]

@swift-ci please smoke test Linux