Add `const` to `dump` method of `OpFoldResult`.

[mravishankar]

Summary

While most dump methods are marked const, some arent marked as
const. Adding const to OpFoldResult here since this was
encountered as an issue while debugging (doing dump within a debug
console threw an error indicating the method should be marked
const).

mlir/include/mlir/IR/OpDefinition.h

 
 raw_ostream &operator<<(raw_ostream &os, OpFoldResult ofr);
 
 /// This class represents a single result from folding an operation.
 class OpFoldResult : public PointerUnion<Attribute, Value> {
   using PointerUnion<Attribute, Value>::PointerUnion;
 
 public:
-  void dump() { llvm::errs() << *this << "\n"; }
+  void dump() const { llvm::errs() << *this << "\n"; }
 };
 
 /// Allow printing to a stream.
 inline raw_ostream &operator<<(raw_ostream &os, OpFoldResult ofr) {
   if (Value value = ofr.dyn_cast<Value>())
     value.print(os);
   else
     ofr.dyn_cast<Attribute>().print(os);