Use StringRef and ArrayRef return values when buf passed.

For functions where a SmallVector is passed to be used as a stack buffer for
the return value, have the return value be the appropriate StringRef or
ArrayRef type.  This allows for both more natural usage and enables directly
returning (rather than copying) a permanently stored or constant string.

src/SinkImpl.h

@@ -28,7 +28,8 @@ class SinkImpl {
   SinkImpl& operator=(const SinkImpl& queue) = delete;
 
   llvm::StringRef GetName() const { return m_name; }
-  virtual void GetDescription(llvm::SmallVectorImpl<char>& desc) const = 0;
+  virtual llvm::StringRef GetDescription(
+      llvm::SmallVectorImpl<char>& buf) const = 0;
 
   void Enable() {
     std::lock_guard<std::mutex> lock(m_mutex);