[native] Stop using @_implementationOnly import in package configs

@_implementationOnly does not work correctly unless Swift’s Library Evolution dialect is enabled — it leads to obscure module loading failures.

There seems to be no way to create a compile-time condition on whether library evolution is enabled, so we need to stop using @_implementationOnly imports in all package configurations.

Author: lorentey

Sources/Atomics/Unmanaged extensions.swift

@@ -16,9 +16,17 @@ 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
+// Note: with ATOMICS_NATIVE_BUILTINS, this file contains the last remaining
+// import of the shims module, and we only need it to get the declarations for
+// _sa_retain_n/_sa_release_n. The import is unfortunately still problematic;
+// these functions need to be moved into the stdlib or (preferably) we need
+// a compiler-level fix for https://bugs.swift.org/browse/SR-13708 to get rid
+// of it.
+//
+// Hiding the import using @_implementationOnly is not possible unless
+// Swift's library evolution dialect is enabled. (Which we cannot easily test
+// here.) Perhaps `internal import` will help work around this at some point.
 import _AtomicsShims
 #endif