Ignore unary constraint tuples during typechecking (#17511)

We deliberately avoid defining a magical `Unit%` class, for reasons
that I have expounded upon in the newly added
`Note [Ignore unary constraint tuples]` in `TcHsType`. However, a
sneaky user could try to insert `Unit%` into their program by way of
Template Haskell, leading to the interface-file error observed
in #17511. To avoid this, any time we encounter a unary constraint
tuple during typechecking, we drop the surrounding constraint tuple
application. This is safe to do since `Unit% a` and `a` would be
semantically equivalent (unlike other forms of unary tuples).

Fixes #17511.

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