[FirestoreSwift] Add FirebaseCore dependency to avoid linker issues (#10319)

* [FirestoreSwift] Add FirebaseCore dependency

* Add note about FirebaseCoreExtension

Author: ncooke3

FirebaseCoreExtension.podspec

@@ -6,6 +6,8 @@ Pod::Spec.new do |s|
     s.description      = <<-DESC
     Not for public use.
     Common FirebaseCore APIs for use in Firebase product SDKs.
+    When depending on `FirebaseCoreExtension`, also depend on `FirebaseCore` to
+    avoid potential linker issues.
                          DESC
 
     s.homepage         = 'https://firebase.google.com'

FirebaseFirestoreSwift.podspec

@@ -34,6 +34,7 @@ Google Cloud Firestore is a NoSQL document database built for automatic scaling,
     'Firestore/Swift/Source/**/*.swift',
   ]
 
+  s.dependency 'FirebaseCore', '~> 10.0'
   s.dependency 'FirebaseCoreExtension', '~> 10.0'
   s.dependency 'FirebaseFirestore', '~> 10.0'
   s.dependency 'FirebaseSharedSwift', '~> 10.0'

Package.swift

@@ -242,6 +242,8 @@ let package = Package(
     // MARK: - Firebase Core Extension
 
     // Extension of FirebaseCore for consuming by Swift product SDKs.
+    // When depending on `FirebaseCoreExtension`, also depend on `FirebaseCore`
+    // to avoid potential linker issues.
     .target(
       name: "FirebaseCoreExtension",
       path: "FirebaseCore/Extension",
@@ -702,6 +704,7 @@ let package = Package(
     .target(
       name: "FirebaseFirestoreSwift",
       dependencies: [
+        "FirebaseCore",
         "FirebaseCoreExtension",
         "FirebaseFirestore",
         "FirebaseSharedSwift",