Make the enter code for small data types panic

Think of this as a counter proposal to #21792, the other end of a spectrum of which we currently reside in the middle.

The suggestion of @clyring in https://github.com/ghc-proposals/ghc-proposals/pull/530 to retain the panicking enter code for ByteArray# and friends and instead give it proper, non-zero tags gave me pause. I tried hard to follow my intuition and make it break, but wasn't able to (we'll revisit my attempts below). Thinking about what is so special about ByteArray#, I concluded "nothing" and propose the following:

Make the enter code of any taggable BoxedRep normal form crash.

(Where "taggable" refers to the architecture specific distinction of "small constructor families", e.g., those that can be given a tag.)

If this is possible, then we'll have a straight win in terms of debugging; if not, we can adapt our reproducer to prove that under Proposal 530 ByteArray# could be legimately entered.

Here's my attempt to break the suggestion:

data B = T | F deriving Show

b :: B
b = <churn>

main = b `seq` print b

Assuming nothing is inlined, the seq will need to enter b, so there are closures of type B that need to be enterable. But it turns out that we only need thunk or indirection closures to be enterable, not the values T or F. When we enter the thunk for churn, we'll run its code and ultimately (call another thunk/indirection/function returning a properly tagged value, or) "see", in the code for churn, that the result is plain T or F, for which the code in churn can be generated in a way that doesn't need to enter T or F and make sure that the proper tags are set.

I'm a bit hazy about indirectee pointers, especially static ones and perhaps blackholes. Consider the example above; after b has been seq'd, it will be evaluated by print again. By then, b no longer points to the thunk object for churn but to an INDirection closure, the payload pointer (indirectee pointer) of which must absolutely be properly tagged. Is that always the case?