[Support] Fix CRTP for GraphWriter (NFC) (#152811)

Commit 474bbc1 forgot to change the members of GraphWriterBase from private 
to protected. 
Because of this, GraphWriter can'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;