[Coroutines] Use std::optional in CoroFrame.cpp (NFC)

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

llvm/lib/Transforms/Coroutines/CoroFrame.cpp

@@ -37,6 +37,7 @@
 #include "llvm/Transforms/Utils/Local.h"
 #include "llvm/Transforms/Utils/PromoteMemToReg.h"
 #include <algorithm>
+#include <optional>
 
 using namespace llvm;
 
@@ -1137,7 +1138,7 @@ static StructType *buildFrameType(Function &F, coro::Shape &Shape,
   FrameTypeBuilder B(C, DL, MaxFrameAlignment);
 
   AllocaInst *PromiseAlloca = Shape.getPromiseAlloca();
-  Optional<FieldIDType> SwitchIndexFieldId;
+  std::optional<FieldIDType> SwitchIndexFieldId;
 
   if (Shape.ABI == coro::ABI::Switch) {
     auto *FramePtrTy = FrameTy->getPointerTo();