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

17 files changed, 2 insertions, 2 deletions

diff --git a/Firestore/Source/API/FIRDocumentChange.m b/Firestore/Source/API/FIRDocumentChange.mm
index d1d9999..d1d9999 100644
--- a/Firestore/Source/API/FIRDocumentChange.m
+++ b/Firestore/Source/API/FIRDocumentChange.mm
diff --git a/Firestore/Source/API/FIRDocumentReference.m b/Firestore/Source/API/FIRDocumentReference.mm
index 05253f7..05253f7 100644
--- a/Firestore/Source/API/FIRDocumentReference.m
+++ b/Firestore/Source/API/FIRDocumentReference.mm
diff --git a/Firestore/Source/API/FIRDocumentSnapshot.m b/Firestore/Source/API/FIRDocumentSnapshot.mm
index 10709e8..10709e8 100644
--- a/Firestore/Source/API/FIRDocumentSnapshot.m
+++ b/Firestore/Source/API/FIRDocumentSnapshot.mm
diff --git a/Firestore/Source/API/FIRFieldPath.m b/Firestore/Source/API/FIRFieldPath.mm
index f4e532f..f4e532f 100644
--- a/Firestore/Source/API/FIRFieldPath.m
+++ b/Firestore/Source/API/FIRFieldPath.mm
diff --git a/Firestore/Source/API/FIRFieldValue.m b/Firestore/Source/API/FIRFieldValue.mm
index 7ae4fb0..7ae4fb0 100644
--- a/Firestore/Source/API/FIRFieldValue.m
+++ b/Firestore/Source/API/FIRFieldValue.mm
diff --git a/Firestore/Source/API/FIRFirestore.m b/Firestore/Source/API/FIRFirestore.mm
index 9df3711..10367bd 100644
--- a/Firestore/Source/API/FIRFirestore.m
+++ b/Firestore/Source/API/FIRFirestore.mm
@@ -41,7 +41,7 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
-NSString *const FIRFirestoreErrorDomain = @"FIRFirestoreErrorDomain";
+extern "C" NSString *const FIRFirestoreErrorDomain = @"FIRFirestoreErrorDomain";
 
 @interface FIRFirestore ()
 
diff --git a/Firestore/Source/API/FIRFirestoreSettings.m b/Firestore/Source/API/FIRFirestoreSettings.mm
index 9677ff6..9677ff6 100644
--- a/Firestore/Source/API/FIRFirestoreSettings.m
+++ b/Firestore/Source/API/FIRFirestoreSettings.mm
diff --git a/Firestore/Source/API/FIRFirestoreVersion.m b/Firestore/Source/API/FIRFirestoreVersion.mm
index 4f8bb28..b1fe480 100644
--- a/Firestore/Source/API/FIRFirestoreVersion.m
+++ b/Firestore/Source/API/FIRFirestoreVersion.mm
@@ -25,5 +25,5 @@
 #define STR(x) STR_EXPAND(x)
 #define STR_EXPAND(x) #x
 
-const unsigned char *const FirebaseFirestoreVersionString =
+extern "C" const unsigned char *const FirebaseFirestoreVersionString =
     (const unsigned char *const)STR(FIRFirestore_VERSION);
diff --git a/Firestore/Source/API/FIRListenerRegistration.m b/Firestore/Source/API/FIRListenerRegistration.mm
index 9f4ddd5..9f4ddd5 100644
--- a/Firestore/Source/API/FIRListenerRegistration.m
+++ b/Firestore/Source/API/FIRListenerRegistration.mm
diff --git a/Firestore/Source/API/FIRQuery.m b/Firestore/Source/API/FIRQuery.mm
index 1bbf91e..1bbf91e 100644
--- a/Firestore/Source/API/FIRQuery.m
+++ b/Firestore/Source/API/FIRQuery.mm
diff --git a/Firestore/Source/API/FIRQuerySnapshot.m b/Firestore/Source/API/FIRQuerySnapshot.mm
index abee84c..abee84c 100644
--- a/Firestore/Source/API/FIRQuerySnapshot.m
+++ b/Firestore/Source/API/FIRQuerySnapshot.mm
diff --git a/Firestore/Source/API/FIRSetOptions.m b/Firestore/Source/API/FIRSetOptions.mm
index b41086c..b41086c 100644
--- a/Firestore/Source/API/FIRSetOptions.m
+++ b/Firestore/Source/API/FIRSetOptions.mm
diff --git a/Firestore/Source/API/FIRSnapshotMetadata.m b/Firestore/Source/API/FIRSnapshotMetadata.mm
index 27747ce..27747ce 100644
--- a/Firestore/Source/API/FIRSnapshotMetadata.m
+++ b/Firestore/Source/API/FIRSnapshotMetadata.mm
diff --git a/Firestore/Source/API/FIRSnapshotOptions.m b/Firestore/Source/API/FIRSnapshotOptions.mm
index 72ea3cc..72ea3cc 100644
--- a/Firestore/Source/API/FIRSnapshotOptions.m
+++ b/Firestore/Source/API/FIRSnapshotOptions.mm
diff --git a/Firestore/Source/API/FIRTransaction.m b/Firestore/Source/API/FIRTransaction.mm
index 5edff19..5edff19 100644
--- a/Firestore/Source/API/FIRTransaction.m
+++ b/Firestore/Source/API/FIRTransaction.mm
diff --git a/Firestore/Source/API/FIRWriteBatch.m b/Firestore/Source/API/FIRWriteBatch.mm
index b1cfa09..b1cfa09 100644
--- a/Firestore/Source/API/FIRWriteBatch.m
+++ b/Firestore/Source/API/FIRWriteBatch.mm
diff --git a/Firestore/Source/API/FSTUserDataConverter.m b/Firestore/Source/API/FSTUserDataConverter.mm
index 414aadb..414aadb 100644
--- a/Firestore/Source/API/FSTUserDataConverter.m
+++ b/Firestore/Source/API/FSTUserDataConverter.mm