Add a couple missing Optional includes

To avoid synced PRs, add some temporary `Optional.h` includes so that
the move from `llvm::Optional` -> `std::optional` in LLVM can be merged.

Once the LLVM change is in we can merge the Swift move. Then we'll need
one final PR to fully remove `Optional.h` and `None.h` (this will likely
have to be a synced PR since optional references keep being added).

Author: bnbarham

include/swift/AST/IRGenOptions.h

@@ -25,6 +25,7 @@
 #include "swift/Basic/OptimizationMode.h"
 #include "swift/Config.h"
 #include "clang/Basic/PointerAuthOptions.h"
+#include "llvm/ADT/Optional.h"
 #include "llvm/IR/CallingConv.h"
 // FIXME: This include is just for llvm::SanitizerCoverageOptions. We should
 // split the header upstream so we don't include so much.

include/swift/Driver/Driver.h

@@ -25,6 +25,7 @@
 #include "swift/Basic/Sanitizers.h"
 #include "swift/Driver/Util.h"
 #include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/Optional.h"
 #include "llvm/ADT/StringRef.h"
 
 #include <functional>