Remove the use of dispatch_once that is heap backed.

Apple recently updated the docs on dispatch_once to point out
that the storage for the dispatch_once_t must be static or global,
but not something that was ever used before as the implementation
doesn't use a memory barrier.  So we drop the use and create the
semaphore when needed and use an atomic swap deal with any
threading races.

objectivec/GPBMessage.m

@@ -738,6 +738,25 @@ void GPBClearMessageAutocreator(GPBMessage *self) {
   self->autocreatorExtension_ = nil;
 }
 
+// Call this before using the readOnlySemaphore_. This ensures it is created only once.
+void GPBPrepareReadOnlySemaphore(GPBMessage *self) {
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdirect-ivar-access"
+#pragma clang diagnostic ignored "-Wdeprecated-declarations"
+
+  // Create the semaphore on demand (rather than init) as developers might not cause them
+  // to be needed, and the heap usage can add up.  The atomic swap is used to avoid needing
+  // another lock around creating it.
+  if (self->readOnlySemaphore_ == nil) {
+    dispatch_semaphore_t worker = dispatch_semaphore_create(1);
+    if (!OSAtomicCompareAndSwapPtrBarrier(NULL, worker, (void * volatile *)&(self->readOnlySemaphore_))) {
+      dispatch_release(worker);
+    }
+  }
+
+#pragma clang diagnostic pop
+}
+
 static GPBUnknownFieldSet *GetOrMakeUnknownFields(GPBMessage *self) {
   if (!self->unknownFields_) {
     self->unknownFields_ = [[GPBUnknownFieldSet alloc] init];

objectivec/GPBMessage_PackagePrivate.h

@@ -70,7 +70,6 @@ typedef struct GPBMessage_Storage *GPBMessage_StoragePtr;
   // Use of readOnlySemaphore_ must be prefaced by a call to
   // GPBPrepareReadOnlySemaphore to ensure it has been created. This allows
   // readOnlySemaphore_ to be only created when actually needed.
-  dispatch_once_t readOnlySemaphoreCreationOnce_;
   dispatch_semaphore_t readOnlySemaphore_;
 }
 
@@ -105,22 +104,7 @@ CF_EXTERN_C_BEGIN
 
 
 // Call this before using the readOnlySemaphore_. This ensures it is created only once.
-NS_INLINE void GPBPrepareReadOnlySemaphore(GPBMessage *self) {
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wdirect-ivar-access"
-
-// Starting on Xcode 8.3, the static analyzer complains that the dispatch_once_t
-// variable passed to dispatch_once should not be allocated on the heap or
-// stack. Given that the semaphore is also an instance variable of the message,
-// both variables are cleared at the same time, so this is safe.
-#if !defined(__clang_analyzer__)
-  dispatch_once(&self->readOnlySemaphoreCreationOnce_, ^{
-    self->readOnlySemaphore_ = dispatch_semaphore_create(1);
-  });
-#endif  // !defined(__clang_analyzer__)
-
-#pragma clang diagnostic pop
-}
+void GPBPrepareReadOnlySemaphore(GPBMessage *self);
 
 // Returns a new instance that was automatically created by |autocreator| for
 // its field |field|.