On 5.9, hide the _AtomicsShims import from the public interface

This appears to resolve a build failure when library evolution is enabled. (This isn’t a fully supported configuration (this package is not ABI stable), but it should still be possible to build the package in that configuration.)

@_implementationOnly has bad failure modes when library evolution isn’t enabled, but I hope we’ll be avoiding those here, as on 5.9+, we’re only using it to import a couple of functions — we aren’t relying on imported things to set struct layouts or anything like that. 🤞

Author: lorentey

Sources/Atomics/Unmanaged extensions.swift

@@ -16,6 +16,8 @@ internal func _sa_retain_n(_ object: UnsafeMutableRawPointer, _ delta: UInt32)
 
 @_silgen_name("_sa_release_n")
 internal func _sa_release_n(_ object: UnsafeMutableRawPointer, _ delta: UInt32)
+#elseif ATOMICS_NATIVE_BUILTINS
+@_implementationOnly import _AtomicsShims
 #else
 import _AtomicsShims
 #endif