Optimize constant arithmetic

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).