[Support] Enable CRTP for GraphWriter (NFC) (#152322)

Previously, specializing the GraphWriter class required a full class
specialization.
This change introduces CRTP for GraphWriter, allowing for partial
specialization.

This change is in support of printing the module dependency graph as
part of the RFC for driver-managed module builds, for which we want to
print the graph nodes in a more human-readable format by:
- Printing descriptive IDs instead of pointer addresses as node labels.
- Printing the full node labels separately from the node relations to
avoid clutter.

With this approach, only GraphWriter::writeNodes() needs to be
specialized (, aside from DOTGraphTraits).

RFC for driver-managed module builds:
https://discourse.llvm.org/t/rfc-modules-support-simple-c-20-modules-use-from-the-clang-driver-without-a-build-system

Author: naveen-seth

llvm/include/llvm/Support/GraphWriter.h

@@ -61,8 +61,7 @@ enum Name {
 LLVM_ABI bool DisplayGraph(StringRef Filename, bool wait = true,
                            GraphProgram::Name program = GraphProgram::DOT);
 
-template<typename GraphType>
-class GraphWriter {
+template <typename GraphType, typename Derived> class GraphWriterBase {
   raw_ostream &O;
   const GraphType &G;
   bool RenderUsingHTML = false;
@@ -75,9 +74,15 @@ class GraphWriter {
   DOTTraits DTraits;
 
   static_assert(std::is_pointer_v<NodeRef>,
-                "FIXME: Currently GraphWriter requires the NodeRef type to be "
-                "a pointer.\nThe pointer usage should be moved to "
-                "DOTGraphTraits, and removed from GraphWriter itself.");
+                "FIXME: Currently GraphWriterBase requires the NodeRef type to "
+                "be a pointer.\nThe pointer usage should be moved to "
+                "DOTGraphTraits, and removed from GraphWriterBase itself.");
+
+  // Cast the 'this' pointer to the derived type and return a reference.
+  Derived &getDerived() { return *static_cast<Derived *>(this); }
+  const Derived &getDerived() const {
+    return *static_cast<const Derived *>(this);
+  }
 
   // Writes the edge labels of the node to O and returns true if there are any
   // edge labels not equal to the empty string "".
@@ -118,23 +123,24 @@ class GraphWriter {
   }
 
 public:
-  GraphWriter(raw_ostream &o, const GraphType &g, bool SN) : O(o), G(g) {
+  GraphWriterBase(raw_ostream &o, const GraphType &g, bool SN) : O(o), G(g) {
     DTraits = DOTTraits(SN);
     RenderUsingHTML = DTraits.renderNodesUsingHTML();
   }
+  virtual ~GraphWriterBase() {}
 
   void writeGraph(const std::string &Title = "") {
     // Output the header for the graph...
-    writeHeader(Title);
+    getDerived().writeHeader(Title);
 
     // Emit all of the nodes in the graph...
-    writeNodes();
+    getDerived().writeNodes();
 
     // Output any customizations on the graph
-    DOTGraphTraits<GraphType>::addCustomGraphFeatures(G, *this);
+    DOTGraphTraits<GraphType>::addCustomGraphFeatures(G, getDerived());
 
     // Output the end of the graph
-    writeFooter();
+    getDerived().writeFooter();
   }
 
   void writeHeader(const std::string &Title) {
@@ -166,8 +172,8 @@ class GraphWriter {
   void writeNodes() {
     // Loop over the graph, printing it out...
     for (const auto Node : nodes<GraphType>(G))
-      if (!isNodeHidden(Node))
-        writeNode(Node);
+      if (!getDerived().isNodeHidden(Node))
+        getDerived().writeNode(Node);
   }
 
   bool isNodeHidden(NodeRef Node) { return DTraits.isNodeHidden(Node, G); }
@@ -302,9 +308,9 @@ class GraphWriter {
       if (DTraits.getEdgeSourceLabel(Node, EI).empty())
         edgeidx = -1;
 
-      emitEdge(static_cast<const void*>(Node), edgeidx,
-               static_cast<const void*>(TargetNode), DestPort,
-               DTraits.getEdgeAttributes(Node, EI, G));
+      getDerived().emitEdge(static_cast<const void *>(Node), edgeidx,
+                            static_cast<const void *>(TargetNode), DestPort,
+                            DTraits.getEdgeAttributes(Node, EI, G));
     }
   }
 
@@ -357,10 +363,17 @@ class GraphWriter {
   }
 };
 
-template<typename GraphType>
+template <typename GraphType>
+class GraphWriter : public GraphWriterBase<GraphType, GraphWriter<GraphType>> {
+public:
+  GraphWriter(raw_ostream &o, const GraphType &g, bool SN)
+      : GraphWriterBase<GraphType, GraphWriter<GraphType>>(o, g, SN) {}
+  ~GraphWriter() override {}
+};
+
+template <typename GraphType>
 raw_ostream &WriteGraph(raw_ostream &O, const GraphType &G,
-                        bool ShortNames = false,
-                        const Twine &Title = "") {
+                        bool ShortNames = false, const Twine &Title = "") {
   // Start the graph emission process...
   GraphWriter<GraphType> W(O, G, ShortNames);