Pattern match exhaustiveness check only suggets constructors from the vanilla COMPLETE set

Due to how getUnmatchedConstructor is implemented, we only suggest unmatched constructors from the vanilla COMPLETE set. Example:

{-# LANGUAGE PatternSynonyms #-}

module Lib where

data T = A | B | C

pattern P = B
{-# COMPLETE A, P #-}

f :: T -> ()
f A = ()

Generated warning:

    Pattern match(es) are non-exhaustive
    In an equation for ‘f’:
        Patterns not matched:
            B
            C
   |
11 | f A = ()
   | ^^^^^^^^

But P would be enough here! It's hard to say what the user wants here. We could just print all different residual COMPLETE sets, I guess.