Revert "use @_implementationOnly"

This reverts commit 4164fb9.

Author: cherylEnkidu

Firestore/Swift/Source/SwiftAPI/SwiftCppAPI.swift

@@ -15,11 +15,11 @@
  */
 
 #if SWIFT_PACKAGE
-  @_implementationOnly import FirebaseFirestoreCpp
+  @_exported import FirebaseFirestoreCpp
 #endif // SWIFT_PACKAGE
 
 public class SwiftCallingCpp {
-  public static func print(_ value: String) {
+  public init(_ value: String) {
     CppInterfaceCalledBySwift.print(std.string(value))
   }
 }

Package.swift

@@ -477,7 +477,10 @@ let package = Package(
       dependencies: [
         "FirebaseFirestoreTarget",
       ],
-      path: "FirebaseCombineSwift/Sources/Firestore"
+      path: "FirebaseCombineSwift/Sources/Firestore",
+      swiftSettings: [
+        .interoperabilityMode(.Cxx), // C++ interoperability setting
+      ]
     ),
     .target(
       name: "FirebaseStorageCombineSwift",
@@ -1172,7 +1175,10 @@ let package = Package(
         "FirebaseStorage",
         .product(name: "nanopb", package: "nanopb"),
       ],
-      path: "SwiftPMTests/swift-test"
+      path: "SwiftPMTests/swift-test",
+      swiftSettings: [
+        .interoperabilityMode(.Cxx), // C++ interoperability setting
+      ]
     ),
     .testTarget(
       name: "analytics-import-test",