[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.

(cherry picked from commit 55bd906906)

lib/Frontend/ArgsToFrontendOptionsConverter.cpp

@@ -206,6 +206,10 @@ bool ArgsToFrontendOptionsConverter::convert(
     Opts.PrintTargetInfo = true;
   }
 
+  if (Args.hasArg(OPT_print_supported_features)) {
+    Opts.PrintSupportedFeatures = true;
+  }
+
   if (const Arg *A = Args.getLastArg(OPT_verify_generic_signatures)) {
     Opts.VerifyGenericSignaturesInModule = A->getValue();
   }