Add more minor enhancements to ItaniumDemangle.h.

Make TemplateArgs::getParams() const and return reference instead of
a copy.

Signed-off-by: Konstantin S Bobrovsky <[email protected]>

Author: kbobrovs

llvm/include/llvm/Demangle/ItaniumDemangle.h

@@ -1199,7 +1199,7 @@ class TemplateArgs final : public Node {
 
   template<typename Fn> void match(Fn F) const { F(Params); }
 
-  NodeArray getParams() { return Params; }
+  const NodeArray &getParams() const { return Params; }
 
   void printLeft(OutputStream &S) const override {
     S += "<";