[NFC] add llvm namespace to Optional and None

This is phase-1 of switching from llvm::Optional to std::optional in the
next rebranch. llvm::Optional was removed from upstream LLVM, so we need
to migrate off rather soon. On Darwin, std::optional, and llvm::Optional
have the same layout, so we don't need to be as concerned about ABI
beyond the name mangling. `llvm::Optional` is only returned from one
function in
```
getStandardTypeSubst(StringRef TypeName,
                     bool allowConcurrencyManglings);
```
It's the return value, so it should not impact the mangling of the
function, and the layout is the same as `std::optional`, so it should be
mostly okay. This function doesn't appear to have users, and the ABI was
already broken 2 years ago for concurrency and no one seemed to notice
so this should be "okay".

I'm doing the migration incrementally so that folks working on main can
cherry-pick back to the release/5.9 branch. Once 5.9 is done and locked
away, then we can go through and finish the replacement. Since `None`
and `Optional` show up in contexts where they are not `llvm::None` and
`llvm::Optional`, I'm preparing the work now by going through and
removing the namespace unwrapping and making the `llvm` namespace
explicit. This should make it fairly mechanical to go through and
replace llvm::Optional with std::optional, and llvm::None with
std::nullopt. It's also a change that can be brought onto the
release/5.9 with minimal impact. This should be an NFC change.

lib/SILOptimizer/Transforms/SILCodeMotion.cpp

@@ -1082,7 +1082,7 @@ cheaperToPassOperandsAsArguments(SILInstruction *First,
   auto *SecondStruct = dyn_cast<StructInst>(Second);
 
   if (!FirstStruct || !SecondStruct)
-    return None;
+    return llvm::None;
 
   assert(FirstStruct->getNumOperands() == SecondStruct->getNumOperands() &&
          FirstStruct->getType() == SecondStruct->getType() &&
@@ -1095,20 +1095,20 @@ cheaperToPassOperandsAsArguments(SILInstruction *First,
     if (FirstStruct->getOperand(i) != SecondStruct->getOperand(i)) {
       // Only track one different operand for now
       if (DifferentOperandIndex)
-        return None;
+        return llvm::None;
       DifferentOperandIndex = i;
     }
   }
 
   if (!DifferentOperandIndex)
-    return None;
+    return llvm::None;
 
   // Found a different operand, now check to see if its type is something
   // cheap enough to sink.
   // TODO: Sink more than just integers.
   SILType ArgTy = FirstStruct->getOperand(*DifferentOperandIndex)->getType();
   if (!ArgTy.is<BuiltinIntegerType>())
-    return None;
+    return llvm::None;
 
   return *DifferentOperandIndex;
 }