[Macros on Darwin] Use device platform paths when building for the simulator

To avoid having duplicated macro implementations in both the device and
simulator platforms, when building for a simulator platform, pass
`-external-plugin-path` that points into the *device* platform. There
is no need to have macro implementations in the simulator platforms.

Implements rdar://112563655.

Author: DougGregor

lib/Driver/DarwinToolChains.cpp

@@ -660,6 +660,15 @@ void toolchains::Darwin::addPlatformSpecificPluginFrontendArgs(
     llvm::sys::path::remove_filename(platformPath); // specific SDK
     llvm::sys::path::remove_filename(platformPath); // SDKs
     llvm::sys::path::remove_filename(platformPath); // Developer
+
+    StringRef platformName = llvm::sys::path::filename(platformPath);
+    if (platformName.endswith("Simulator.platform")){
+      StringRef devicePlatformName =
+          platformName.drop_back(strlen("Simulator.platform"));
+      llvm::sys::path::remove_filename(platformPath); // Platform
+      llvm::sys::path::append(platformPath, devicePlatformName + "OS.platform");
+    }
+
     llvm::sys::path::append(platformPath, "Developer");
     addExternalPluginFrontendArgs(platformPath, inputArgs, arguments);
   }