[Dynamic Casting] Use old boxing semantics for pre-Fall-2023 apps

For Apple platforms, enable the new stricter boxing semantics only
for apps built against the Fall 2023 or later SDK.  This
avoids breaking apps that relied on the old behavior and
have not been updated since then.

Author: tbkka

stdlib/public/runtime/Bincompat.cpp

@@ -61,6 +61,11 @@ static enum sdk_test isAppAtLeastSpring2021() {
     const dyld_build_version_t spring_2021_os_versions = {0xffffffff, 0x007e50301};
     return isAppAtLeast(spring_2021_os_versions);
 }
+
+static enum sdk_test isAppAtLeastFall2023() {
+    const dyld_build_version_t fall_2023_os_versions = {0xffffffff, 0x007e70901};
+    return isAppAtLeast(fall_2023_os_versions);
+}
 #endif
 
 static _SwiftStdlibVersion binCompatVersionOverride = { 0 };
@@ -189,7 +194,11 @@ bool useLegacyOptionalNilInjectionInCasting() {
 // by that protocol.
 bool useLegacyObjCBoxingInCasting() {
 #if BINARY_COMPATIBILITY_APPLE
-  return false; // For now, always use the new behavior on Apple OSes
+  switch (isAppAtLeastFall2023()) {
+  case oldOS: return true; // Legacy behavior on old OS
+  case oldApp: return true; // Legacy behavior for old apps
+  case newApp: return false; // New behavior for new apps
+  }
 #else
   return false; // Always use the new behavior on non-Apple OSes
 #endif
@@ -209,7 +218,11 @@ bool useLegacyObjCBoxingInCasting() {
 
 bool useLegacySwiftValueUnboxingInCasting() {
 #if BINARY_COMPATIBILITY_APPLE
-  return false; // For now, always use the new behavior on Apple OSes
+  switch (isAppAtLeastFall2023()) {
+  case oldOS: return true; // Legacy behavior on old OS
+  case oldApp: return true; // Legacy behavior for old apps
+  case newApp: return false; // New behavior for new apps
+  }
 #else
   return false; // Always use the new behavior on non-Apple OSes
 #endif