Misc spot fixes

- Ensure QuotedString prints characters as `char` and not as an integer-like `unsigned char`
- Assert when trying to add a null child node to a Demangle::Node

Author: jckarter

lib/Basic/QuotedString.cpp

@@ -36,7 +36,7 @@ void swift::printAsQuotedString(llvm::raw_ostream &out, llvm::StringRef text) {
         };
         out << "\\u{" << hexdigit[c >> 4] << hexdigit[c & 0xF] << '}';
       } else {
-        out << c;
+        out << (char)c;
       }
       break;
     }

lib/Demangling/Demangler.cpp

@@ -317,6 +317,7 @@ Node::iterator Node::end() const {
 }
 
 void Node::addChild(NodePointer Child, NodeFactory &Factory) {
+  assert(Child);
   switch (NodePayloadKind) {
     case PayloadKind::None:
       InlineChildren[0] = Child;