firebase-ios-sdk

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.