In immediate mode, detect the host OS version on Apple platforms.

This allows script mode to pick up the current version of macOS
instead of defaulting to 10.9, making it unnecessary to write #available.
A -target flag can still override this if you're trying to write a
portable script.

The logic is a little tortured to avoid having to actually link to
Foundation.framework or libobjc.

Finishes rdar://problem/29433205.

lib/Frontend/CompilerInvocation.cpp

@@ -12,6 +12,10 @@
 
 #include "swift/Frontend/Frontend.h"
 
+#if __APPLE__
+# include "AppleHostVersionDetection.h"
+#endif
+
 #include "swift/Strings.h"
 #include "swift/AST/DiagnosticsFrontend.h"
 #include "swift/Basic/Platform.h"
@@ -904,6 +908,21 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     Target = llvm::Triple(A->getValue());
     TargetArg = A->getValue();
   }
+#if __APPLE__
+  else if (FrontendOpts.actionIsImmediate()) {
+    clang::VersionTuple currentOSVersion = inferAppleHostOSVersion();
+    if (currentOSVersion.getMajor() != 0) {
+      llvm::Triple::OSType currentOS = Target.getOS();
+      if (currentOS == llvm::Triple::Darwin)
+        currentOS = llvm::Triple::MacOSX;
+
+      SmallString<16> newOSBuf;
+      llvm::raw_svector_ostream newOS(newOSBuf);
+      newOS << llvm::Triple::getOSTypeName(currentOS) << currentOSVersion;
+      Target.setOSName(newOS.str());
+    }
+  }
+#endif
 
   Opts.EnableObjCInterop = Target.isOSDarwin();
   if (auto A = Args.getLastArg(OPT_enable_objc_interop,