Optimize constant arithmetic

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Some function translations apply arithmetic to arguments/return values; when the operands happen to be constants, we can simply compute the result in post-translation and embed the result constant.

See #28031 for an example with SQL Server's string.IndexOf.

It may be a good idea to do some non-arithmatic evaluation as well (IIRC there's some logical optimization happening in SqlNullabilityProcessor, which may make more sense in a general optimizing visitor or something).

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Note: we've starting some optimizations in SqlExpressionFactory, when expressions are originally constructed; this applies the optimization very early (good), possibly unlocking other optimizations. Constant arithmetic evaluation probably falls under this umbrella too: if the operands of the unary/binary expression happen to be constants, just evaluate and return the response.

/cc @ranma42