[mlir][NFC] Fix `GpuKernelOutliningPass` copy constructor warnings

[unterumarmung]

Summary

1. Call copy constructor of the base class
2. Assign value of the option directly

mlir/lib/Dialect/GPU/Transforms/KernelOutlining.cpp

class GpuKernelOutliningPass
     : public GpuKernelOutliningBase<GpuKernelOutliningPass> {
 public:
   GpuKernelOutliningPass(StringRef dlStr) {
     if (!dlStr.empty() && !dataLayoutStr.hasValue())
       dataLayoutStr = dlStr.str();
   }
 
   GpuKernelOutliningPass(const GpuKernelOutliningPass &other)
-      : dataLayoutSpec(other.dataLayoutSpec) {
-    dataLayoutStr = other.dataLayoutStr;
+      : GpuKernelOutliningBase(other), dataLayoutSpec(other.dataLayoutSpec) {
+    dataLayoutStr = other.dataLayoutStr.getValue();
   }
 
   LogicalResult initialize(MLIRContext *context) override {
     // Initialize the data layout specification from the data layout string.
     if (!dataLayoutStr.empty()) {
       Attribute resultAttr = mlir::parseAttribute(dataLayoutStr, context);
       if (!resultAttr)
         return failure();