[LLVM] Use std::nullopt instead of llvm::None (apache#13617)

Pass `std::nullopt` to initialization of `PassBuilder` for `PGOOptions`.
LLVM is moving away from its own `Optional` type to `std::optional`.

src/target/llvm/codegen_llvm.cc

@@ -362,7 +362,7 @@ void CodeGenLLVM::Optimize() {
 
   llvm::PipelineTuningOptions pto = llvm::PipelineTuningOptions();
   llvm::PassInstrumentationCallbacks pic;
-  llvm::PassBuilder builder(tm, pto, llvm::None, &pic);
+  llvm::PassBuilder builder(tm, pto, std::nullopt, &pic);
 
   llvm::LoopAnalysisManager lam;
   llvm::FunctionAnalysisManager fam;