Codegen: Always prepare filesystem

Summary:
For any Pod that uses the codegen, create references to code-gen'd files in local filesystem regardless of Pod install status by invoking the same command used by `prepare_command` whenever `pod install` is run.

This works around the issue where CocoaPods may decide to skip running `prepare_command`. While this is expected CocoaPods behavior, external factors may result in the deletion of the original code-gen'd files in which case we need to make sure that running `pod install` will bring these files back.

See Test Plan for more details on how to reproduce the issue being fixed.

Fixes T97404254.

Changelog:

[Internal] Codegen invoked with every `pod install` regardless of pod install status

Differential Revision: D30116640

fbshipit-source-id: 81db5dff1d4c4f8ae22b5dbe822609c770789ac8

scripts/react_native_pods.rb

@@ -193,7 +193,9 @@ def use_react_native_codegen!(spec, options={})
   end
 
   # Prepare filesystem by creating empty files that will be picked up as references by CocoaPods.
-  spec.prepare_command = "mkdir -p #{generated_dirs.join(" ")} && touch #{generated_files.join(" ")}"
+  prepare_command = "mkdir -p #{generated_dirs.join(" ")} && touch -a #{generated_files.join(" ")}"
+  system(prepare_command) # Always run prepare_command when a podspec uses the codegen, as CocoaPods may skip invoking this command in certain scenarios
+  spec.prepare_command = prepare_command
 
   spec.script_phase = {
     :name => 'Generate Specs',