[Reader] Use std::optional in BitcodeReader.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/Bitcode/Reader/BitcodeReader.cpp

@@ -80,6 +80,7 @@
 #include <deque>
 #include <map>
 #include <memory>
+#include <optional>
 #include <set>
 #include <string>
 #include <system_error>
@@ -593,7 +594,7 @@ class BitcodeReader : public BitcodeReaderBase, public GVMaterializer {
   /// ValueList must be destroyed before Alloc.
   BumpPtrAllocator Alloc;
   BitcodeReaderValueList ValueList;
-  Optional<MetadataLoader> MDLoader;
+  std::optional<MetadataLoader> MDLoader;
   std::vector<Comdat *> ComdatList;
   DenseSet<GlobalObject *> ImplicitComdatObjects;
   SmallVector<Instruction *, 64> InstructionList;
@@ -3205,7 +3206,7 @@ Error BitcodeReader::parseConstants() {
         PointeeType = getTypeByID(Record[OpNum++]);
 
       bool InBounds = false;
-      Optional<unsigned> InRangeIndex;
+      std::optional<unsigned> InRangeIndex;
       if (BitCode == bitc::CST_CODE_CE_GEP_WITH_INRANGE_INDEX) {
         uint64_t Op = Record[OpNum++];
         InBounds = Op & 1;