fix: add 'squash' to 'norm_cast' attribute for 'Int.cast_negSucc' (#8365

)

This is an attempt at fixing the following behavior of `norm_cast`.
```lean

example (n : ℤ) : (-37 : ℤ) = n := by
  norm_cast -- goal is `Int.negSucc 36 = n`
  sorry
```
See [this discussion](https://leanprover.zulipchat.com/#narrow/stream/287929-mathlib4/topic/linarith.20fails.20in.20a.20simple.20example/near/401592369).

Mathlib/Data/Int/Cast/Basic.lean

@@ -49,7 +49,7 @@ namespace Int
 
 variable {R : Type u} [AddGroupWithOne R]
 
-@[simp, norm_cast]
+@[simp, norm_cast squash]
 theorem cast_negSucc (n : ℕ) : (-[n+1] : R) = -(n + 1 : ℕ) :=
   AddGroupWithOne.intCast_negSucc n
 #align int.cast_neg_succ_of_nat Int.cast_negSuccₓ