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

# Conflicts:
#	React-Core.podspec
#	packages/rn-tester/Podfile.lock

RNTester/Podfile.lock

@@ -489,7 +489,7 @@ SPEC CHECKSUMS:
   React: e1c65dd41cb9db13b99f24608e47dd595f28ca9a
   React-ART: 2ec4583a9a32a52e271db84c5ebaf8dfdf1b0c57
   React-callinvoker: 552a6a6bc8b3bb794cf108ad59e5a9e2e3b4fc98
-  React-Core: 9d341e725dc9cd2f49e4c49ad1fc4e8776aa2639
+  React-Core: a42b4deaadbde5a2d8e2dab7a2b874303f733cf4
   React-CoreModules: 5335e168165da7f7083ce7147768d36d3e292318
   React-cxxreact: d3261ec5f7d11743fbf21e263a34ea51d1f13ebc
   React-jsi: 54245e1d5f4b690dec614a73a3795964eeef13a8

React-Core.podspec

@@ -49,7 +49,7 @@ 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)/Folly\"" }
+  s.pod_target_xcconfig    = { "HEADER_SEARCH_PATHS" => "\"$(PODS_TARGET_SRCROOT)/ReactCommon\" \"$(PODS_ROOT)/boost-for-react-native\" \"$(PODS_ROOT)/DoubleConversion\" \"$(PODS_ROOT)/Folly\"", "DEFINES_MODULE" => "YES" }
   s.user_target_xcconfig   = { "HEADER_SEARCH_PATHS" => "\"$(PODS_ROOT)/Headers/Private/React-Core\""}
   s.default_subspec        = "Default"