Allow ReactCommon/Folly to be used in Swift libraries (DEFINES_MODULE) (#43327)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/43327

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## Summary

Swift Pods require the use of [modular headers](https://blog.cocoapods.org/CocoaPods-1.5.0/) to be statically linked. To interop with Objective-C modules, you need to make the Objective-C module "define a Module", that is modular header export.

This is already the case for a few podspecs so they can be consumed in Swift libraries, but `ReactCommon` and `RCT-Folly` don't do this yet and therefore this breaks in a few libraries of mine, for example see this issue: https://github.com/mrousavy/react-native-vision-camera/issues/195.

If I were to include `ReactCommon` or `RCT-Folly` in my Swift library's podspec, the following error arises:

```
[!] The following Swift pods cannot yet be integrated as static libraries:

The Swift pod `VisionCamera` depends upon `RCT-Folly`, which does not define modules.
To opt into those targets generating module maps (which is necessary to import them from Swift
when building as static libraries), you may set `use_modular_headers!` globally in your Podfile, or
specify `:modular_headers => true` for particular dependencies.
```

So this PR fixes this issue by allowing Swift libraries to consume the `ReactCommon` and `RCT-Folly` podspecs since they now export modular headers.

## Changelog

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[General] [Fixed] - Expose Modular Headers for `ReactCommon` podspec
[General] [Fixed] - Expose Modular Headers for `RCT-Folly` podspec

Pull Request resolved: https://github.com/facebook/react-native/pull/31858

Test Plan: * Add s.dependency "ReactCommon" or RCT-Folly to a Swift pod and see what happens. (See https://github.com/mrousavy/react-native-vision-camera/pull/273)

Reviewed By: dmytrorykun

Differential Revision: D54539127

Pulled By: cipolleschi

fbshipit-source-id: 2291cc0c8d6675521b220b02ef0c3c6a3e73be38

packages/react-native/ReactCommon/ReactCommon.podspec

@@ -36,6 +36,7 @@ Pod::Spec.new do |s|
   s.compiler_flags         = folly_compiler_flags + ' ' + boost_compiler_flags
   s.pod_target_xcconfig    = { "HEADER_SEARCH_PATHS" => "\"$(PODS_ROOT)/boost\" \"$(PODS_ROOT)/RCT-Folly\" \"$(PODS_ROOT)/DoubleConversion\" \"$(PODS_ROOT)/fmt/include\" \"$(PODS_ROOT)/Headers/Private/React-Core\"",
                                "USE_HEADERMAP" => "YES",
+                               "DEFINES_MODULE" => "YES",
                                "CLANG_CXX_LANGUAGE_STANDARD" => "c++20",
                                "GCC_WARN_PEDANTIC" => "YES" }
   if ENV['USE_FRAMEWORKS']

packages/react-native/third-party-podspecs/RCT-Folly.podspec

@@ -81,6 +81,7 @@ Pod::Spec.new do |spec|
                         'folly/system/*.h',
   spec.libraries           = "c++abi" # NOTE Apple-only: Keep c++abi here due to https://github.com/react-native-community/releases/issues/251
   spec.pod_target_xcconfig = { "USE_HEADERMAP" => "NO",
+                               "DEFINES_MODULE" => "YES",
                                "CLANG_CXX_LANGUAGE_STANDARD" => "c++20",
                                "HEADER_SEARCH_PATHS" => "\"$(PODS_TARGET_SRCROOT)\" \"$(PODS_ROOT)/boost\" \"$(PODS_ROOT)/DoubleConversion\" \"$(PODS_ROOT)/fmt/include\"",
                                # In dynamic framework (use_frameworks!) mode, ignore the unused and undefined boost symbols when generating the library.