Make React-Core compatible with Swift modules (#29995)

Summary:
Related to #29633

Support Swift based libraries using Xcode 12’s build system.

## Changelog

[iOS] [Fixed] - Support Swift based libraries using Xcode 12’s build system.

Pull Request resolved: #29995

Test Plan:
* Building RNTester still works
* Swift based pod tested in mrousavy/react-native-blurhash#58

Reviewed By: fkgozali

Differential Revision: D23824438

Pulled By: appden

fbshipit-source-id: 418caf9808cb6326e3d6efdc8b37131a5705e7f6

React-Core.podspec

@@ -48,7 +48,7 @@ Pod::Spec.new do |s|
   s.header_dir             = "React"
   s.framework              = "JavaScriptCore"
   s.library                = "stdc++"
-  s.pod_target_xcconfig    = { "HEADER_SEARCH_PATHS" => "\"$(PODS_TARGET_SRCROOT)/ReactCommon\" \"$(PODS_ROOT)/boost-for-react-native\" \"$(PODS_ROOT)/DoubleConversion\" \"$(PODS_ROOT)/RCT-Folly\"" }
+  s.pod_target_xcconfig    = { "HEADER_SEARCH_PATHS" => "\"$(PODS_TARGET_SRCROOT)/ReactCommon\" \"$(PODS_ROOT)/boost-for-react-native\" \"$(PODS_ROOT)/DoubleConversion\" \"$(PODS_ROOT)/RCT-Folly\"", "DEFINES_MODULE" => "YES" }
   s.user_target_xcconfig   = { "HEADER_SEARCH_PATHS" => "\"$(PODS_ROOT)/Headers/Private/React-Core\""}
   s.default_subspec        = "Default"
 

packages/rn-tester/Podfile.lock

@@ -505,7 +505,7 @@ SPEC CHECKSUMS:
   RCTTypeSafety: 84db212a990ce622a28f1bcb1ac68c658e722373
   React: cafb3c2321f7df55ce90dbf29d513799a79e4418
   React-callinvoker: 0dada022d38b73e6e15b33e2a96476153f79bbf6
-  React-Core: d377a770bb13aa5120a6ce553f75f0e1cbc1aafe
+  React-Core: fbbfaa3cbb1ca720fd74ef938214b5943da1aa5d
   React-CoreModules: f38b671f8df4c1c744ed69f00264539a7c4024b4
   React-cxxreact: de6de17eac6bbaa4f9fad46b66e7f0c4aaaf863d
   React-jsi: 652ad7cb7ff8c87e0e9fb11e9ebcbbc70cdfe865