When resolving anonymous dot notation (`.ident x y z`), it would reduce
the expected type to whnf. Now, it unfolds definitions step-by-step,
even if the type synonym is for a pi type like so
```lean
def Foo : Prop := ∀ a : Nat, a = a
protected theorem Foo.intro : Foo := sorry
example : Foo := .intro
```
Closes#4761
