llvm-undname: Correctly demangle vararg parameters

FunctionSignatureNode already had an IsVariadic field,
but it wasn't used anywhere yet. Set it and use it.

llvm-svn: 362541
diff --git a/llvm/lib/Demangle/MicrosoftDemangleNodes.cpp b/llvm/lib/Demangle/MicrosoftDemangleNodes.cpp
index c07fde8..63ca475 100644
--- a/llvm/lib/Demangle/MicrosoftDemangleNodes.cpp
+++ b/llvm/lib/Demangle/MicrosoftDemangleNodes.cpp
@@ -414,6 +414,12 @@
       Params->output(OS, Flags);
     else
       OS << "void";
+
+    if (IsVariadic) {
+      if (OS.back() != '(')
+        OS << ", ";
+      OS << "...";
+    }
     OS << ")";
   }