[GSYM] Use std::optional in FunctionInfo.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/DebugInfo/GSYM/FunctionInfo.cpp

@@ -12,6 +12,7 @@
 #include "llvm/DebugInfo/GSYM/LineTable.h"
 #include "llvm/DebugInfo/GSYM/InlineInfo.h"
 #include "llvm/Support/DataExtractor.h"
+#include <optional>
 
 using namespace llvm;
 using namespace gsym;
@@ -178,8 +179,8 @@ llvm::Expected<LookupResult> FunctionInfo::lookup(DataExtractor &Data,
         Offset - 4);
   LR.FuncName = GR.getString(NameOffset);
   bool Done = false;
-  Optional<LineEntry> LineEntry;
-  Optional<DataExtractor> InlineInfoData;
+  std::optional<LineEntry> LineEntry;
+  std::optional<DataExtractor> InlineInfoData;
   while (!Done) {
     if (!Data.isValidOffsetForDataOfSize(Offset, 8))
       return createStringError(std::errc::io_error,