Refactor and clean up the CMake build ahead of CMake/iOS support (#5051)

* Use early exit in objc_framework CMakeLists.txt

* Clean up objc_framework CMakeLists.txt

  * Use `version` consistently
  * Use `firebase_ios_version` for the repo-level version
  * Remove INCLUDE directories that are no longer needed
  * Sort dependencies

* Simplify Example/App/CMakeLists.txt

  * Add `OBJC_FLAGS` directly, avoiding a separate `sources` variable.
  * Add resources directly

* Rename test host app to firebase_firestore_example_app

* Add a firebase_ios prefix to functions and variables

This reduces the chance of collision with other projects.

* Simplify compiler_setup

Now that compiler_setup.cmake no longer has side-effects, fold
compiler_id.cmake and archive_options.cmake into it and move
the include of compiler_setup up to the top of the main build.

Firestore/Example/Tests/API/CMakeLists.txt

@@ -1,4 +1,4 @@
-# Copyright 2019 Google
+# Copyright 2019 Google LLC
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -13,9 +13,9 @@
 # limitations under the License.
 
 if(APPLE)
-  objc_test(
+  firebase_ios_objc_test(
     firebase_firestore_objc_api_test
-    HOST firestore_example_app_macos
+    HOST firebase_firestore_example_app
     SOURCES
       FIRCollectionReferenceTests.mm
       FIRDocumentReferenceTests.mm