Move all Firestore Objective-C to Objective-C++ (#734)

* Move all Firestore files to Objective-C++
* Update project file references
* Don't use module imports from Objective-C++
* Use extern "C" for C-accessible globals
* Work around more stringent type checking in Objective-C++
  * NSMutableDictionary ivars aren't implicitly casted to NSDictionary
  * FSTMaybeDocument callback can't be passed a function that accepts
    FSTDocument
  * NSComparisonResult can't be multiplied by -1 without casting
* Add a #include <inttypes.h> where needed
* Avoid using C++ keywords as variables
* Remove #if __cplusplus guards

9 files changed, 33 insertions, 21 deletions

diff --git a/Firestore/Source/Core/FSTDatabaseInfo.m b/Firestore/Source/Core/FSTDatabaseInfo.mm
index 2dbe61a..2dbe61a 100644
--- a/Firestore/Source/Core/FSTDatabaseInfo.m
+++ b/Firestore/Source/Core/FSTDatabaseInfo.mm
diff --git a/Firestore/Source/Core/FSTEventManager.m b/Firestore/Source/Core/FSTEventManager.mm
index bc204a0..bc204a0 100644
--- a/Firestore/Source/Core/FSTEventManager.m
+++ b/Firestore/Source/Core/FSTEventManager.mm
diff --git a/Firestore/Source/Core/FSTFirestoreClient.m b/Firestore/Source/Core/FSTFirestoreClient.mm
index fff644d..fff644d 100644
--- a/Firestore/Source/Core/FSTFirestoreClient.m
+++ b/Firestore/Source/Core/FSTFirestoreClient.mm
diff --git a/Firestore/Source/Core/FSTListenSequence.m b/Firestore/Source/Core/FSTListenSequence.mm
index 6f50d35..6f50d35 100644
--- a/Firestore/Source/Core/FSTListenSequence.m
+++ b/Firestore/Source/Core/FSTListenSequence.mm
diff --git a/Firestore/Source/Core/FSTQuery.m b/Firestore/Source/Core/FSTQuery.mm
index 13657f7..8c98687 100644
--- a/Firestore/Source/Core/FSTQuery.m
+++ b/Firestore/Source/Core/FSTQuery.mm
@@ -27,6 +27,13 @@ NS_ASSUME_NONNULL_BEGIN
 
 #pragma mark - FSTRelationFilterOperator functions
 
+/**
+ * Returns the reverse order (i.e. Ascending => Descending) etc.
+ */
+static constexpr NSComparisonResult ReverseOrder(NSComparisonResult result) {
+  return static_cast<NSComparisonResult>(-static_cast<NSInteger>(result));
+}
+
 NSString *FSTStringFromQueryRelationOperator(FSTRelationFilterOperator filterOperator) {
   switch (filterOperator) {
     case FSTRelationFilterOperatorLessThan:
@@ -287,16 +294,20 @@ NSString *FSTStringFromQueryRelationOperator(FSTRelationFilterOperator filterOpe
 #pragma mark - Public methods
 
 - (NSComparisonResult)compareDocument:(FSTDocument *)document1 toDocument:(FSTDocument *)document2 {
-  int modifier = self.isAscending ? 1 : -1;
+  NSComparisonResult result;
   if ([self.field isEqual:[FSTFieldPath keyFieldPath]]) {
-    return (NSComparisonResult)(modifier * FSTDocumentKeyComparator(document1.key, document2.key));
+    result = FSTDocumentKeyComparator(document1.key, document2.key);
   } else {
     FSTFieldValue *value1 = [document1 fieldForPath:self.field];
     FSTFieldValue *value2 = [document2 fieldForPath:self.field];
     FSTAssert(value1 != nil && value2 != nil,
               @"Trying to compare documents on fields that don't exist.");
-    return modifier * [value1 compare:value2];
+    result = [value1 compare:value2];
+  }
+  if (!self.isAscending) {
+    result = ReverseOrder(result);
   }
+  return result;
 }
 
 - (NSString *)canonicalID {
@@ -377,7 +388,8 @@ NSString *FSTStringFromQueryRelationOperator(FSTRelationFilterOperator filterOpe
     if ([sortOrderComponent.field isEqual:[FSTFieldPath keyFieldPath]]) {
       FSTAssert([fieldValue isKindOfClass:[FSTReferenceValue class]],
                 @"FSTBound has a non-key value where the key path is being used %@", fieldValue);
-      comparison = [fieldValue.value compare:document.key];
+      FSTReferenceValue *refValue = (FSTReferenceValue *)fieldValue;
+      comparison = [refValue.value compare:document.key];
     } else {
       FSTFieldValue *docValue = [document fieldForPath:sortOrderComponent.field];
       FSTAssert(docValue != nil, @"Field should exist since document matched the orderBy already.");
@@ -385,7 +397,7 @@ NSString *FSTStringFromQueryRelationOperator(FSTRelationFilterOperator filterOpe
     }
 
     if (!sortOrderComponent.isAscending) {
-      comparison = comparison * -1;
+      comparison = ReverseOrder(comparison);
     }
 
     if (comparison != 0) {
diff --git a/Firestore/Source/Core/FSTSnapshotVersion.m b/Firestore/Source/Core/FSTSnapshotVersion.mm
index 980ae52..980ae52 100644
--- a/Firestore/Source/Core/FSTSnapshotVersion.m
+++ b/Firestore/Source/Core/FSTSnapshotVersion.mm
diff --git a/Firestore/Source/Core/FSTTransaction.m b/Firestore/Source/Core/FSTTransaction.mm
index c4c5f27..f97888a 100644
--- a/Firestore/Source/Core/FSTTransaction.m
+++ b/Firestore/Source/Core/FSTTransaction.mm
@@ -104,22 +104,22 @@ NS_ASSUME_NONNULL_BEGIN
     FSTThrowInvalidUsage(@"FIRIllegalStateException",
                          @"All reads in a transaction must be done before any writes.");
   }
-  [self.datastore
-      lookupDocuments:keys
-           completion:^(NSArray<FSTDocument *> *_Nullable documents, NSError *_Nullable error) {
-             if (error) {
-               completion(nil, error);
-               return;
-             }
-             for (FSTMaybeDocument *doc in documents) {
-               NSError *recordError = nil;
-               if (![self recordVersionForDocument:doc error:&recordError]) {
-                 completion(nil, recordError);
-                 return;
-               }
-             }
-             completion(documents, nil);
-           }];
+  [self.datastore lookupDocuments:keys
+                       completion:^(NSArray<FSTMaybeDocument *> *_Nullable documents,
+                                    NSError *_Nullable error) {
+                         if (error) {
+                           completion(nil, error);
+                           return;
+                         }
+                         for (FSTMaybeDocument *doc in documents) {
+                           NSError *recordError = nil;
+                           if (![self recordVersionForDocument:doc error:&recordError]) {
+                             completion(nil, recordError);
+                             return;
+                           }
+                         }
+                         completion(documents, nil);
+                       }];
 }
 
 /** Stores mutations to be written when commitWithCompletion is called. */
diff --git a/Firestore/Source/Core/FSTView.m b/Firestore/Source/Core/FSTView.mm
index d6b4558..d6b4558 100644
--- a/Firestore/Source/Core/FSTView.m
+++ b/Firestore/Source/Core/FSTView.mm
diff --git a/Firestore/Source/Core/FSTViewSnapshot.m b/Firestore/Source/Core/FSTViewSnapshot.mm
index e60b785..e60b785 100644
--- a/Firestore/Source/Core/FSTViewSnapshot.m
+++ b/Firestore/Source/Core/FSTViewSnapshot.mm