Note [Unify with expected type before typechecking arguments]

Consider this (#19364)
  data Pair a b = Pair a b
  baz :: MkPair Int Bool
  baz = MkPair "yes" "no"

We instantiate MkPair with `alpha`, `beta`, and push its argument
types (`alpha` and `beta`) into the arguments ("yes" and "no").
But if we first unify the result type (Pair alpha beta) with the expected
type (Pair Int Bool) we will push the much more informative types
`Int` and `Bool` into the arguments.   This makes a difference:

Unify result type /after/ typechecking the args
    • Couldn't match type ‘[Char]’ with ‘Bool’
      Expected type: Pair Foo Bar
        Actual type: Pair [Char] [Char]
    • In the expression: Pair "yes" "no"

Unify result type /before/ typechecking the args
    • Couldn't match type ‘[Char]’ with ‘Bool’
      Expected: Foo
        Actual: String
    • In the first argument of ‘Pair’, namely ‘"yes"’

The latter is much better. That is why we call checkResultType before tcValArgs.