[stdlib] Restore the _copy function.

Deprecate it in swift 6.

Author: nate-chandler

stdlib/public/core/LifetimeManager.swift

@@ -265,3 +265,12 @@ extension String {
   }
 }
 #endif
+
+@available(*, deprecated, message: "Use the copy operator")
+@_alwaysEmitIntoClient
+@inlinable
+@_transparent
+@_semantics("lifetimemanagement.copy")
+public func _copy<T>(_ value: T) -> T {
+  copy value
+}

test/stdlib/LifetimeManagement.swift

@@ -8,6 +8,11 @@ class Klass {}
 
 var suite = TestSuite("LifetimeManagement")
 
+suite.test("_copy") {
+  let k = Klass()
+  expectTrue(k === _copy(k))
+}
+
 suite.test("copy") {
   let k = Klass()
   expectTrue(k === copy k)