[IR] llvm::Optional => std::optional

Many llvm/IR/* files have been migrated by other contributors.
This migrates most remaining files.

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -551,7 +551,7 @@ public:
     return makeArrayRef(getTrailingObjects<unsigned>(), NumOperands);
   }
 
-  Optional<unsigned> getInRangeIndex() const {
+  std::optional<unsigned> getInRangeIndex() const {
     assert(Opcode == Instruction::GetElementPtr);
     if (Extra == (unsigned)-1)
       return std::nullopt;