cmXMLWriter: Add RAII helpers to allow DOM-like syntax

Use RAII for cmXMLWriter::StartElement/EndElement to make nesting
automatic.

Source/cmXMLWriter.h

@@ -133,4 +133,56 @@ private:
   bool IsContent;
 };
 
+class cmXMLElement; // IWYU pragma: keep
+
+class cmXMLDocument
+{
+public:
+  cmXMLDocument(cmXMLWriter& xml)
+    : xmlwr(xml)
+  {
+    xmlwr.StartDocument();
+  }
+  ~cmXMLDocument() { xmlwr.EndDocument(); }
+private:
+  friend class cmXMLElement;
+  cmXMLWriter& xmlwr;
+};
+
+class cmXMLElement
+{
+public:
+  cmXMLElement(cmXMLWriter& xml, const char* tag)
+    : xmlwr(xml)
+  {
+    xmlwr.StartElement(tag);
+  }
+  cmXMLElement(cmXMLElement const& par, const char* tag)
+    : xmlwr(par.xmlwr)
+  {
+    xmlwr.StartElement(tag);
+  }
+  cmXMLElement(cmXMLDocument const& doc, const char* tag)
+    : xmlwr(doc.xmlwr)
+  {
+    xmlwr.StartElement(tag);
+  }
+  ~cmXMLElement() { xmlwr.EndElement(); }
+
+  template <typename T>
+  cmXMLElement& Attribute(const char* name, T const& value)
+  {
+    xmlwr.Attribute(name, value);
+    return *this;
+  }
+  template <typename T>
+  void Content(T const& content)
+  {
+    xmlwr.Content(content);
+  }
+
+private:
+  cmXMLWriter& xmlwr;
+};
+
 #endif