Fix warnings exposed by Xcode 9.1 in Auth (#350)

Author: paulb777

Firebase/Auth/Source/FIRAuth.m

@@ -892,7 +892,7 @@ - (void)sendPasswordResetWithNullableActionCodeSettings:(nullable FIRActionCodeS
   });
 }
 
-- (BOOL)signOut:(NSError *_Nullable *_Nullable)error {
+- (BOOL)signOut:(NSError *_Nullable __autoreleasing *_Nullable)error {
   __block BOOL result = YES;
   dispatch_sync(FIRAuthGlobalWorkQueue(), ^{
     if (!_currentUser) {

Firebase/Auth/Source/FIRAuthGlobalWorkQueue.h

@@ -26,6 +26,6 @@ NS_ASSUME_NONNULL_BEGIN
         implementations that may involve contested code shall dispatch to this work queue as the
         first thing they do.
  */
-extern dispatch_queue_t FIRAuthGlobalWorkQueue();
+extern dispatch_queue_t FIRAuthGlobalWorkQueue(void);
 
 NS_ASSUME_NONNULL_END

Firebase/Auth/Source/FIRAuthURLPresenter.m

@@ -157,7 +157,7 @@ - (void)finishPresentationWithURL:(nullable NSURL *)URL
   _UIDelegate = nil;
   FIRAuthURLPresentationCompletion completion = _completion;
   _completion = nil;
-  void (^finishBlock)() = ^() {
+  void (^finishBlock)(void) = ^() {
     _isPresenting = NO;
     completion(URL, error);
   };

Firebase/Core/FIRAppAssociationRegistration.m

@@ -20,7 +20,7 @@ @implementation FIRAppAssociationRegistration
 
 + (nullable id)registeredObjectWithHost:(id)host
                                     key:(NSString *)key
-                          creationBlock:(id _Nullable (^)())creationBlock {
+                          creationBlock:(id _Nullable (^)(void))creationBlock {
   @synchronized(self) {
     SEL dictKey = @selector(registeredObjectWithHost:key:creationBlock:);
     NSMutableDictionary<NSString *, id> *objectsByKey = objc_getAssociatedObject(host, dictKey);

Firebase/Core/Private/FIRAppAssociationRegistration.h

@@ -41,7 +41,7 @@ NS_ASSUME_NONNULL_BEGIN
  */
 + (nullable ObjectType)registeredObjectWithHost:(id)host
                                             key:(NSString *)key
-                                  creationBlock:(ObjectType _Nullable (^)())creationBlock;
+                                  creationBlock:(ObjectType _Nullable (^)(void))creationBlock;
 
 @end