[SPARK-18999][SQL][MINOR] simplify Literal codegen

## What changes were proposed in this pull request?

`Literal` can use `CodegenContex.addReferenceObj` to implement codegen, instead of `CodegenFallback`.  This can also simplify the generated code a little bit, before we will generate: `((Expression) references[1]).eval(null)`, now it's just `references[1]`.

## How was this patch tested?

N/A

Author: Wenchen Fan <wenchen@databricks.com>

Closes #16402 from cloud-fan/minor.