[Support] Fix CRTP for GraphWriter (NFC)

Commit 474bbc1 forgot to change GraphWriterBase's members from private
to protected. Because of this, GraphWriter cant’t be properly
partially specialized using CRTP.
This commit corrects that, allowing GraphWriterBase to be partially
specialized using CRTP as intended.

Author: naveen-seth

llvm/include/llvm/Support/GraphWriter.h

@@ -62,6 +62,7 @@ LLVM_ABI bool DisplayGraph(StringRef Filename, bool wait = true,
                            GraphProgram::Name program = GraphProgram::DOT);
 
 template <typename GraphType, typename Derived> class GraphWriterBase {
+protected:
   raw_ostream &O;
   const GraphType &G;
   bool RenderUsingHTML = false;