Pass -Xfrontend -mergeable-symbols for embedded objects #8648
Description
Description
Currently SwiftPM links intermediate swift objects when producing a product in embedded swift. This is not correct, and results in linker errors about multiple definitions. Changing SwiftPM to not pass multiple objects is not worth doing in SwiftPM's native build system as it will be replaced swift-build, so as a stop-gap solution we should pass -Xfrontend -mergeable-symbols when producing these intermediate swift objects to allow the linker to dedup them.
Expected behavior
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Actual behavior
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Steps to reproduce
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Swift Package Manager version/commit hash
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Swift & OS version (output of swift --version && uname -a)
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