missing_asserts_for_indexing: support comparing to const values

[andjsrk]

Fixes #16237

I think current behavior is unsatisfying because comparing v.len() with const values in assert!() is pretty common (31.2k results on GitHub).
This PR would make enabling this lint less painful in my opinion.

Example

#![warn(clippy::missing_asserts_for_indexing)]

const MIN_LEN: usize = 2;
fn assert_cmp_to_const(v: &[u8]) {
    assert!(v.len() >= MIN_LEN);

    let _ = v[0] + v[1]; // was warning, now ok
    let _ = v[0] + v[1] + v[2]; // correctly warned
}

---

changelog: [missing_asserts_for_indexing]: support comparing to const values

[rustbot]

r? @llogiq

rustbot has assigned @llogiq.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[andjsrk]

I have changed suggesting behavior as below, but I'm not sure if it is reasonable:

if all of these were satisfied, suggest v.len() > N_ORIGINAL_EXPR rather than v.len() >= N_PLUS_ONE_LITERAL:

• v.len() is compared to a const value.
• the operator in the assertion is either <= or >=.
• the highest index is equal to asserted length.