Make -fbyte-code prevent unboxed tuples/sums from implying object code (#16876)

This resolves #16876 by making the explicit use of `-fbyte-code`
prevent code that enables `UnboxedTuples` or `UnboxedSums` from
automatically compiling to object code. This allows for a nice
middle ground where most code that enables `UnboxedTuples`/-`Sums`
will still benefit from automatically enabling `-fobject-code`, but
allows power users who wish to avoid this behavior in certain corner
cases (such as `lens`, whose use case is documented in #16876) to do
so.

Along the way, I did a little cleanup of the relevant code and
documentation:

* `enableCodeGenForUnboxedTuples` was only checking for the presence
  of `UnboxedTuples`, but `UnboxedSums` has the same complications.
  I fixed this and renamed the function to
  `enableCodeGenForUnboxedTuplesOrSums`.
* I amended the users' guide with a discussion of these issues.

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