Add a basic "re-mangler", which turns a parsed

demangling tree back into a mangled string.

Also, extend the demangling API in a few obvious
ways, and simplify testing for failure in the
node-returning APIs by having them simply return
null instead of a spurious Failure node.

Also, preserve slightly more information in the
demangling tree.  The goal here is eventually to
always allow a perfect round-trip through the
demangler parse tree.  This patch gets us close,
but we're not quite there yet.

Tests to follow.

Swift SVN r24473

lib/Basic/DemangleWrappers.cpp

@@ -32,6 +32,10 @@ static void printNode(llvm::raw_ostream &Out, const Node *node,
                       unsigned depth) {
   // Indent two spaces per depth.
   Out.indent(depth * 2);
+  if (!node) {
+    Out << "<<NULL>>";
+    return;
+  }
   Out << "kind=" << getNodeKindString(node->getKind());
   if (node->hasText()) {
     Out << ", text=\"" << node->getText() << '\"';