[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/RemoteAST/RemoteAST.cpp

@@ -91,7 +91,7 @@ public:
 /// The template subclasses do target-specific logic.
 class RemoteASTContextImpl {
   std::unique_ptr<IRGenContext> IRGen;
-  Optional<Failure> CurFailure;
+  llvm::Optional<Failure> CurFailure;
 
 public:
   RemoteASTContextImpl() = default;
@@ -391,7 +391,7 @@ private:
   }
 
   /// Attempt to discover the size and alignment of the given type.
-  Optional<std::pair<Size, Alignment>>
+  llvm::Optional<std::pair<Size, Alignment>>
   getTypeSizeAndAlignment(irgen::IRGenModule &IGM, SILType eltTy) {
     auto &eltTI = IGM.getTypeInfo(eltTy);
     if (auto fixedTI = dyn_cast<irgen::FixedTypeInfo>(&eltTI)) {
@@ -400,7 +400,7 @@ private:
     }
 
     // TODO: handle resilient types
-    return None;
+    return llvm::None;
   }
 };