[Frontend] Add -print-supported-features option

This is a replacement for `-emit-supported-features` that prints
all of the upcoming/experimental features supported by the compiler
with some additional meta information in JSON format to stdout.

lib/FrontendTool/FrontendTool.cpp

@@ -46,6 +46,7 @@
 #include "swift/Basic/PrettyStackTrace.h"
 #include "swift/Basic/SourceManager.h"
 #include "swift/Basic/Statistic.h"
+#include "swift/Basic/SupportedFeatures.h"
 #include "swift/Basic/TargetInfo.h"
 #include "swift/Basic/UUID.h"
 #include "swift/Basic/Version.h"
@@ -2063,6 +2064,11 @@ int swift::performFrontend(ArrayRef<const char *> Args,
     return finishDiagProcessing(0, /*verifierEnabled*/ false);
   }
 
+  if (Invocation.getFrontendOptions().PrintSupportedFeatures) {
+    swift::features::printSupportedFeatures(llvm::outs());
+    return finishDiagProcessing(0, /*verifierEnabled*/ false);
+  }
+
   if (Invocation.getFrontendOptions().RequestedAction ==
       FrontendOptions::ActionType::NoneAction) {
     Instance->getDiags().diagnose(SourceLoc(),