Avoid `readability-function-size` clang-tidy checks on `Stringify()` (#5933)

This currently triggers due to preprocessing.

toolchain/sem_ir/stringify.cpp

@@ -669,6 +669,7 @@ class Stringifier {
 
 }  // namespace
 
+// NOLINTNEXTLINE(readability-function-size)
 static auto Stringify(const File& sem_ir, StepStack& step_stack)
     -> std::string {
   RawStringOstream out;