Change the operating system name from OSX to MacOS everywhere

Summary:
- Changed the name in all build artifacts
- Everywhere in C++

Reviewers: #swift_pilot_client, kbasan

Reviewed By: #swift_pilot_client, kbasan

Subscribers: jenkins

Maniphest Tasks: T205

Differential Revision: https://dev.swift-project.org/D69

src/blackconfig/buildconfig.h

@@ -80,8 +80,8 @@ namespace BlackConfig
         //! Windows 10
         static bool isRunningOnWindows10();
 
-        //! Running on Mac OS X platform?
-        static bool isRunningOnMacOSXPlatform();
+        //! Running on MacOS platform?
+        static bool isRunningOnMacOSPlatform();
 
         //! Running on Linux platform?
         static bool isRunningOnLinuxPlatform();