make UI test OS-agnostic

the internal representation of `std::sync::Mutex` depends on the compilation target. due to this,
the compiler produces different number of errors for UI test
`issue-17431-6.rs` depending on the compilation target.

for example, when compiling the UI test to an `*-apple-*` or `*-qnx7*` target, the "cycle detected"
error is not reported

``` console
$ cat src/lib.rs
use std::sync::Mutex;

enum Foo {
    X(Mutex<Option<Foo>>),
}

impl Foo {
    fn bar(self) {}
}

fn main() {}

$ cargo check --target x86_64-apple-ios 2>&1 | rg '^error\['
error[E0072]: recursive type `Foo` has infinite size
```

whereas rustc produces two errors for other OSes, like Linux, which is what the UI test expects

``` console
$ cargo check --target x86_64-unknown-linux-gnu 2>&1 | rg '^error\['
error[E0072]: recursive type `Foo` has infinite size
error[E0391]: cycle detected when computing when `Foo` needs drop
```

this commit replaces the problematic `Mutex` with `UnsafeCell`, which has the same internal
representation regardless of the compilation target. with that change, rustc reports two errors for
all compilation targets.

``` console
$ cat src/lib.rs
use std::cell::UnsafeCell;

enum Foo {
    X(UnsafeCell<Option<Foo>>),
}

impl Foo {
    fn bar(self) {}
}

fn main() {}

$ cargo check --target x86_64-apple-ios 2>&1 | rg '^error\['
error[E0072]: recursive type `Foo` has infinite size
error[E0391]: cycle detected when computing when `Foo` needs drop
```

with this change, we can remove the `ignore-apple` directive as the UI test now also passes on apple
targets.

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