Note [Unquantified tyvars in a pattern synonym]

Consider (#21479)

   data T a where MkT :: Int -> T Char   -- A GADT
   foo :: forall b. Bool -> T b          -- Somewhat strange type

   pattern T1 <- (foo -> MkT)

In the view pattern, foo is instantiated, let's say b :-> b0
where b0 is a unification variable.  Then matching the GADT
MkT will add the "provided" constraint b0~Char, so we might infer
   pattern T1 :: () => (b0~Char) => Int -> Bool

Nothing constrains that `b0`. We don't want to quantify over it.
We don't want to to zonk to Any (we don't like Any showing up in
user-visible types).  So we want to error here. See
Note [Error on unconstrained meta-variables] in GHC.Tc.Utils.TcMType

Hence the call to doNotQuantifyTyVars here.