`large_stack_arrays` still lint on nested const items

[flip1995]

bool is not sufficient to keep track if we're in a constant item:

rust-clippy/clippy_lints/src/large_stack_arrays.rs

Line 33 in 8e60f14

is_in_const_item: bool,

const A: u8 = {
    const B: () = ();

    let x = [0u8; 20_000_000];
    x[0]
};

This will still lint the array assigned to x. But if one comments out const B, it won't lint, because of the fix added in #13485

To fix this, turn the bool into a counter, increment it when entering an item and decrementing it when exiting it.

The current impl also won't prevent the lint from triggering in const fns. But I'm not sure if this is intended intended.

Originally posted by @flip1995 in #13485 (comment)

This issue has been assigned to @GnomedDev via this comment.

[flip1995]

@rustbot assign @GnomedDev

[flip1995]

Well, that didn't work 😅