[Clang][NFC] Adding note on details that are not immediately obvious (#138349)

This code was flagged by static analysis. It was a false positive but
the reason why this code is valid is subtle and folks refactoring this
code in the future could easily miss it.

Author: shafik

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -1137,6 +1137,9 @@ void Sema::PrintInstantiationStack(InstantiationContextDiagFuncRef DiagFunc) {
       // FIXME: For synthesized functions that are not defaulted,
       // produce a note.
       auto *FD = dyn_cast<FunctionDecl>(Active->Entity);
+      // Note: if FD is nullptr currently setting DFK to DefaultedFunctionKind()
+      // will ensure that DFK.isComparison() is false. This is important because
+      // we will uncondtionally dereference FD in the else if.
       DefaultedFunctionKind DFK =
           FD ? getDefaultedFunctionKind(FD) : DefaultedFunctionKind();
       if (DFK.isSpecialMember()) {