Previously, function aliases (ex. combining a `#group` and `#count`)
would make queries non-retryable because `Function#as` would always wrap
a given alias in a non-retryable `SqlLiteral`.

This could be addressed by making `Function#as` behave similarly to
`AliasPredication` (retry everything). However, `Function#as` can also
just be removed so that `Function` uses `AliasPredication#as` instead.
This behaves slightly differently because `AliasPredication#as` will now
return the `Function` wrapped in an `Alias` (meaning the return value
must be used). Only a single place in Active Record needed to be changed
and all others already did the correct thing.

This also resolves a long standing FIXME suggesting removing the custom
`Function#as` in favor of using `Alias` nodes. The other half of the
comment suggesting `Function` could inherit from `Unary` does not make
sense because SQL functions could potentially take multiple parameters,
so the existing superclass should not be changed.