Why is NoExt inhabited?

[Ben Gamari]

Hi Shayan,

During code review a contributor asked about a TTG extension point.
While answering his question I realized I didn't have a good explanation
for why NoExt is inhabited; as far as I can tell it should rather be
unhabited.

If there is a reason for this then can you:

 1. describe why this is so in #16863 [1]
 2. open an MR documenting the reason in Note [Trees that Grow].

If not, perhaps we should consider changing it; the current state of
things is a bit awkward.

Cheers,

- Ben


[1] https://gitlab.haskell.org/ghc/ghc/issues/16863
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