message: Mark XlaBuilder ConstantLiteral method virtual
  This will overridden in MlirHloBuilder in a later change.
diff: @@ -380,7 +380,7 @@ class XlaBuilder {
  return Parameter(parameter_number, shape, name, empty_bools);
  }
  - XlaOp ConstantLiteral(const LiteralSlice& literal);
  + virtual XlaOp ConstantLiteral(const LiteralSlice& literal);
  XlaOp Broadcast(XlaOp operand, absl::Span<const int64> broadcast_sizes);
  