Add Swift extension as a separate pod

Summary:
Adds swift extension file as a separate subspec, as it is not required for the objc projects.

Also this fixes the current issue where the flipper sample app fails to build on xcode 11, as it fails to link the Swift specific literals.

Fixes this too #565

Reviewed By: SidharthGuglani

Differential Revision: D18373993

fbshipit-source-id: 0c058886a837c7ceafcd0167f878b3e0f7763aa1

YogaKit.podspec

@@ -5,7 +5,7 @@
 
 podspec = Pod::Spec.new do |spec|
   spec.name = 'YogaKit'
-  spec.version = '1.14.0'
+  spec.version = '1.17.0'
   spec.license =  { :type => 'MIT', :file => "LICENSE" }
   spec.homepage = 'https://facebook.github.io/yoga/'
   spec.documentation_url = 'https://facebook.github.io/yoga/docs/'
@@ -22,11 +22,20 @@ podspec = Pod::Spec.new do |spec|
   spec.platform = :ios
   spec.ios.deployment_target = '8.0'
   spec.ios.frameworks = 'UIKit'
+  spec.default_subspec = "Core"
   spec.dependency 'Yoga', '~> 1.14'
-  spec.source_files = 'YogaKit/Source/*.{h,m,swift}'
-  spec.public_header_files = 'YogaKit/Source/{YGLayout,UIView+Yoga}.h'
-  spec.private_header_files = 'YogaKit/Source/YGLayout+Private.h'
-  spec.swift_version = '4.0'
+  spec.module_name = 'YogaKit'
+
+  spec.subspec "Core" do |ss|
+    ss.source_files = 'YogaKit/Source/*.{h,m}'
+    ss.public_header_files = 'YogaKit/Source/{YGLayout,UIView+Yoga}.h'
+    ss.private_header_files = 'YogaKit/Source/YGLayout+Private.h'
+  end
+
+  spec.subspec 'SwiftExtension' do |ss|
+    ss.source_files = 'YogaKit/Source/*.{swift}'
+  end
+
 end
 
 # See https://github.com/facebook/yoga/pull/366