stdlib: fix availability for internal COW checks.

Those checks should only be done if linked with a built library, but not when linked with the OS libraries.

Author: eeckstein

stdlib/public/core/Builtin.swift

@@ -346,12 +346,18 @@ internal func _class_getInstancePositiveExtentSize(_ theClass: AnyClass) -> Int
 }
 
 #if INTERNAL_CHECKS_ENABLED
-@available(macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0, *)
+// "9999" means: enable if linked with a built library, but not when linked with
+// the OS libraries.
+// Note: this must not be changed to a "real" OS version.
+@available(macOS 9999, iOS 9999, tvOS 9999, watchOS 9999, *)
 @usableFromInline
 @_silgen_name("_swift_isImmutableCOWBuffer")
 internal func _swift_isImmutableCOWBuffer(_ object: AnyObject) -> Bool
 
-@available(macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0, *)
+// "9999" means: enable if linked with a built library, but not when linked with
+// the OS libraries.
+// Note: this must not be changed to a "real" OS version.
+@available(macOS 9999, iOS 9999, tvOS 9999, watchOS 9999, *)
 @usableFromInline
 @_silgen_name("_swift_setImmutableCOWBuffer")
 internal func _swift_setImmutableCOWBuffer(_ object: AnyObject, _ immutable: Bool) -> Bool

stdlib/public/core/ContiguousArrayBuffer.swift

@@ -13,7 +13,10 @@
 import SwiftShims
 
 #if INTERNAL_CHECKS_ENABLED
-@available(macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0, *)
+// "9999" means: enable if linked with a built library, but not when linked with
+// the OS libraries.
+// Note: this must not be changed to a "real" OS version.
+@available(macOS 9999, iOS 9999, tvOS 9999, watchOS 9999, *)
 @_silgen_name("swift_COWChecksEnabled")
 public func _COWChecksEnabled() -> Bool
 #endif
@@ -460,15 +463,21 @@ internal struct _ContiguousArrayBuffer<Element>: _ArrayBufferProtocol {
   @_alwaysEmitIntoClient
   internal var isImmutable: Bool {
     get {
-      if #available(macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0, *) {
+      // "9999" means: enable if linked with a built library, but not when
+      // linked with the OS libraries.
+      // Note: this must not be changed to a "real" OS version.
+      if #available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *) {
         if (_COWChecksEnabled()) {
           return capacity == 0 || _swift_isImmutableCOWBuffer(_storage)
         }
       }
       return true
     }
     nonmutating set {
-      if #available(macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0, *) {
+      // "9999" means: enable if linked with a built library, but not when
+      // linked with the OS libraries.
+      // Note: this must not be changed to a "real" OS version.
+      if #available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *) {
         if (_COWChecksEnabled()) {
           // Make sure to not modify the empty array singleton (which has a
           // capacity of 0).
@@ -489,7 +498,10 @@ internal struct _ContiguousArrayBuffer<Element>: _ArrayBufferProtocol {
 
   @_alwaysEmitIntoClient
   internal var isMutable: Bool {
-    if #available(macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0, *) {
+      // "9999" means: enable if linked with a built library, but not when
+      // linked with the OS libraries.
+      // Note: this must not be changed to a "real" OS version.
+    if #available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *) {
       if (_COWChecksEnabled()) {
         return !_swift_isImmutableCOWBuffer(_storage)
       }