[get_unwrap]: include a borrow in the suggestion if argument is not an integer literal

[y21]

Fixes #9909

I have to say, I don't really understand what the previous logic was trying to do, but this fixes the linked bug.
It was checking if the argument passed to .get() can be parsed as a usize (i.e. if it's an integer literal, probably?), and if not, it wouldn't include a borrow? I don't know how we came to that conclusion, but that logic doesn't work:

let slice = &[1, 2];
let _r: &i32 = slice.get({ 1 }).unwrap();
// previous suggestion: slice[{ 1 }]
// the suggestion should be: &slice[{ 1 }]

Here the argument passed to it isn't an integer literal, but it should still include a borrow, because it would otherwise change the type from &i32 to i32.

The exception is that if the parent of the get().unwrap() expr is a dereference or a method call or the like, we don't need an explicit borrow because it's automatically inserted by the compiler

changelog: [get_unwrap]: include a borrow in the suggestion if argument is not an integer literal

[rustbot]

r? @giraffate

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[giraffate]

@bors r+

Thanks!

[bors]

📌 Commit bdb2a17 has been approved by giraffate

It is now in the queue for this repository.

[bors]

⌛ Testing commit bdb2a17 with merge 6ec2388...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: giraffate
Pushing 6ec2388 to master...