[LLVM] Fix compilation failure due to minor change

This is just a minor fix where the recent [PR apache#16425](apache#16425) seems
to have missed this change for LLVM 18 and above, and so we're running
into a compilaion failure.

src/target/llvm/llvm_instance.cc

@@ -311,7 +311,7 @@ LLVMTargetInfo::LLVMTargetInfo(LLVMInstance& instance, const TargetJSON& target)
   }
 #else
   if (maybe_level.defined()) {
-    int level = maybe_level.value()->value;
+    int level = maybe_level->value;
     if (level <= 0) {
       opt_level_ = llvm::CodeGenOptLevel::None;
     } else if (level == 1) {