[Concurrency] Hide async_Main from other object files

[kateinoigakukun]

Multiple executable targets with asyc entrypoints within a single package were not able to be tested due to the following linkage failure.
The sync main entrypoint is usually referenced by crt, so it should be visible from the outside of the object file and linked image unit. Therefore, the sync main entrypoints are renamed using -entry-point-function-name by SwiftPM to avoid conflicting with other executable targets.
On the other hand, the async async_Main entrypoint is only referenced from the sync main, so it doesn't need to be visible from other object files. Therefore, we can just hide it from other object files to avoid static link-time conflicts.

The async_Main symbol was defined as hidden SIL visibility, but hidden is still visible from other object files (not visible from other image unit), so this PR makes it private, which corresponds to internal LLVM visibility.

Resolves rdar://114231968

$ ../swift-DEVELOPMENT-SNAPSHOT-2023-10-09-a-ubuntu20.04/usr/bin/swift test
Building for debugging...
error: link command failed with exit code 1 (use -v to see invocation)
/usr/bin/ld.gold: error: /tmp/tmp.o984OA4fPZ/spm-async-main-repro/.build/x86_64-unknown-linux-gnu/debug/b.build/B.swift.o: multiple definition of 'async_Main'
/usr/bin/ld.gold: /tmp/tmp.o984OA4fPZ/spm-async-main-repro/.build/x86_64-unknown-linux-gnu/debug/a.build/A.swift.o: previous definition here
/usr/bin/ld.gold: error: /tmp/tmp.o984OA4fPZ/spm-async-main-repro/.build/x86_64-unknown-linux-gnu/debug/b.build/B.swift.o: multiple definition of 'async_MainTu'
/usr/bin/ld.gold: /tmp/tmp.o984OA4fPZ/spm-async-main-repro/.build/x86_64-unknown-linux-gnu/debug/a.build/A.swift.o: previous definition here
clang-15: error: linker command failed with exit code 1 (use -v to see invocation)
error: fatalError
[39/40] Linking spm-async-main-reproPackageTests.xctest
error: fatalError

let package = Package(
    name: "spm-async-main-repro",
    dependencies: [ ],
    targets: [
        .executableTarget(name: "a", dependencies: []),
        .executableTarget(name: "b", dependencies: []),

        .testTarget(name: "reproTests", dependencies: ["a", "b"]),
    ]
)

[kateinoigakukun]

@swift-ci Please build toolchain

[kateinoigakukun]

Please test with following pull request:
swiftlang/swift-package-manager#6986

@swift-ci Please smoke test

[neonichu]

Thanks @kateinoigakukun 🥳

[etcwilde]

We're seeing test failures here. I believe the symbol should be both private linkage and have hidden visibility.

https://ci.swift.org/job/swift-PR-Linux/11278/

/usr/bin/ld.gold: error: /tmp/Miscellaneous_TestableAsyncExe.itEwsM/Miscellaneous_TestableAsyncExe/.build/x86_64-unknown-linux-gnu/debug/TestableAsyncExe2.build/main.swift.o: multiple definition of 'async_MainTu'
/usr/bin/ld.gold: /tmp/Miscellaneous_TestableAsyncExe.itEwsM/Miscellaneous_TestableAsyncExe/.build/x86_64-unknown-linux-gnu/debug/TestableAsyncExe1.build/main.swift.o: previous definition here
/usr/bin/ld.gold: error: /tmp/Miscellaneous_TestableAsyncExe.itEwsM/Miscellaneous_TestableAsyncExe/.build/x86_64-unknown-linux-gnu/debug/TestableAsyncExe2.build/main.swift.o: multiple definition of 'async_Main'
/usr/bin/ld.gold: /tmp/Miscellaneous_TestableAsyncExe.itEwsM/Miscellaneous_TestableAsyncExe/.build/x86_64-unknown-linux-gnu/debug/TestableAsyncExe1.build/main.swift.o: previous definition here
/usr/bin/ld.gold: error: /tmp/Miscellaneous_TestableAsyncExe.itEwsM/Miscellaneous_TestableAsyncExe/.build/x86_64-unknown-linux-gnu/debug/TestableAsyncExe3.build/NonMain.swift.o: multiple definition of 'async_Main'
/usr/bin/ld.gold: /tmp/Miscellaneous_TestableAsyncExe.itEwsM/Miscellaneous_TestableAsyncExe/.build/x86_64-unknown-linux-gnu/debug/TestableAsyncExe1.build/main.swift.o: previous definition here
/usr/bin/ld.gold: error: /tmp/Miscellaneous_TestableAsyncExe.itEwsM/Miscellaneous_TestableAsyncExe/.build/x86_64-unknown-linux-gnu/debug/TestableAsyncExe3.build/NonMain.swift.o: multiple definition of 'async_MainTu'
/usr/bin/ld.gold: /tmp/Miscellaneous_TestableAsyncExe.itEwsM/Miscellaneous_TestableAsyncExe/.build/x86_64-unknown-linux-gnu/debug/TestableAsyncExe1.build/main.swift.o: previous definition here
/usr/bin/ld.gold: error: /tmp/Miscellaneous_TestableAsyncExe.itEwsM/Miscellaneous_TestableAsyncExe/.build/x86_64-unknown-linux-gnu/debug/TestableAsyncExe4.build/NonMain.swift.o: multiple definition of 'async_Main'
/usr/bin/ld.gold: /tmp/Miscellaneous_TestableAsyncExe.itEwsM/Miscellaneous_TestableAsyncExe/.build/x86_64-unknown-linux-gnu/debug/TestableAsyncExe1.build/main.swift.o: previous definition here
/usr/bin/ld.gold: error: /tmp/Miscellaneous_TestableAsyncExe.itEwsM/Miscellaneous_TestableAsyncExe/.build/x86_64-unknown-linux-gnu/debug/TestableAsyncExe4.build/NonMain.swift.o: multiple definition of 'async_MainTu'
/usr/bin/ld.gold: /tmp/Miscellaneous_TestableAsyncExe.itEwsM/Miscellaneous_TestableAsyncExe/.build/x86_64-unknown-linux-gnu/debug/TestableAsyncExe1.build/main.swift.o: previous definition here
clang-15: error: linker command failed with exit code 1 (use -v to see invocation)
<unknown>:0: error: link command failed with exit code 1 (use -v to see invocation)

[etcwilde]

Actually, scratch that. The above link failure is on the 5.10 branch in a SwiftPM test.
I think this needs cherry-picking into the 5.10 branch. SwiftPM is still automerging changes from main to release/5.10, but Swift is not, so the tests copied into SwiftPM in swiftlang/swift-package-manager@f258cd5 are failing.