Merge pull request #25870 from pschuh/cpp-1

Add -enable-cxx-interop flag and support for extern "C" {}

lib/Frontend/CompilerInvocation.cpp

@@ -499,6 +499,7 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     TargetArg = A->getValue();
   }
 
+  Opts.EnableCXXInterop |= Args.hasArg(OPT_enable_cxx_interop);
   Opts.EnableObjCInterop =
       Args.hasFlag(OPT_enable_objc_interop, OPT_disable_objc_interop,
                    Target.isOSDarwin());