Runtime: Work around rdar://problem/18950072 to avoid paying for .cxx_destruct on Swift objects.

Clang overzealously infects SwiftObject with the HasCXXStructors bit because it contained a struct (with trivial constructor). Manually explode the struct to avoid this.

Swift SVN r23259

stdlib/runtime/SwiftObject.mm

@@ -83,11 +83,22 @@ struct SwiftObject_s {
   long refCount  __attribute__((unavailable));
 };
 
+static_assert(std::is_trivially_constructible<SwiftObject_s>::value,
+              "SwiftObject must be trivially constructible");
+static_assert(std::is_trivially_destructible<SwiftObject_s>::value,
+              "SwiftObject must be trivially destructible");
+
 #if __has_attribute(objc_root_class)
 __attribute__((objc_root_class))
 #endif
 @interface SwiftObject<NSObject> {
-  SwiftObject_s magic;
+  // FIXME: rdar://problem/18950072 Clang emits ObjC++ classes as having
+  // non-trivial structors if they contain any struct fields at all, regardless of
+  // whether they in fact have nontrivial default constructors. Dupe the body
+  // of SwiftObject_s into here as a workaround because we don't want to pay
+  // the cost of .cxx_destruct method dispatch at deallocation time.
+  void *magic_isa  __attribute__((unavailable));
+  long magic_refCount  __attribute__((unavailable));
 }
 
 - (BOOL)isEqual:(id)object;

test/1_stdlib/NSObject.swift

@@ -216,3 +216,36 @@ println("done ValueLike hashValue")
 // CHECK-NEXT: sh2 20
 // CHECK-NEXT: sh3 10
 // CHECK-NEXT: done ValueLike hashValue
+
+// Native Swift objects should not have nontrivial structors from ObjC's point
+// of view.
+class NativeSwift {}
+class GenericNativeSwift<T> {}
+
+var native: AnyObject = NativeSwift()
+
+if native.respondsToSelector(".cxx_construct") {
+  println("SwiftObject has nontrivial constructor")
+} else {
+  println("no nontrivial constructor") // CHECK-NEXT: no nontrivial constructor
+}
+if native.respondsToSelector(".cxx_destruct") {
+  println("SwiftObject has nontrivial destructor")
+} else {
+  println("no nontrivial destructor") // CHECK-NEXT: no nontrivial destructor
+}
+
+native = GenericNativeSwift<Int>()
+
+if native.respondsToSelector(".cxx_construct") {
+  println("SwiftObject has nontrivial constructor")
+} else {
+  println("no nontrivial constructor") // CHECK-NEXT: no nontrivial constructor
+}
+if native.respondsToSelector(".cxx_destruct") {
+  println("SwiftObject has nontrivial destructor")
+} else {
+  println("no nontrivial destructor") // CHECK-NEXT: no nontrivial destructor
+}
+
+