Concatenate contiguous literal arguments of `CONCAT` during the expression simplification.

[HaoYang670]

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For an expression concat(col1, 'a', 'b', col2, 'd', 'f', ...) we can simplify it to concat(col1, 'ab', col2, 'df ...)`

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1. Ignore all null literals
2. cast all literals to UTF8
3. concatenate contiguous literals.

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We could not do this.

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