Add rationale for rustc_type_ir behind allowing rustc::potential-query-instability lint

[ismailarilik]

Context: #84447

Motivation:

1. This lint asks to do so. An example:

   error: using `values` can result in unstable query results
      --> compiler/rustc_query_system/src/dep_graph/serialized.rs:654:50
       |
   654 |             let mut stats: Vec<_> = record_stats.values().collect();
       |                                                  ^^^^^^
       |
       = note: if you believe this case to be fine, allow this lint and add a comment explaining your rationale

2. This would prevent further fix attempts so wastes of time as well.
3. Nearly all of those allowances has a comment which explains the rationale.

To do: Add a comment explaning the rationale behind allowing rustc::potential-query-instability lint for all occurrences in the file compiler/rustc_type_ir/src/search_graph/mod.rs. You can find the places where this lint is triggered by searching with #[allow(rustc::potential_query_instability)] in this file.

[ismailarilik]

@rustbot label C-cleanup T-compiler

[compiler-errors]

It would be nice if you actually linked the specific callsites where this lint is triggering in the crate.

[ismailarilik]

I was just thinking that how that kind of allow statement looks like:

#[allow(rustc::potential_query_instability, "Order does not matter here.")]

I wished it to exist when I searched for these allow statements again and saw that a new statement is added without any reasoning. Of course, I've just ignored it because it is obvious that someone put it there for a reason.

But no one can ensure that this person didn't miss anything. So a reasoning would always be helpful. So this proposition may be a way to enforce to put a reasoning before allowing a lint.

For now, I am just putting this here. If you don't think I am crazy, I will open an issue and propose this.

[ismailarilik]

I was just thinking that how that kind of allow statement looks like:

#[allow(rustc::potential_query_instability, "Order does not matter here.")]

Oh, I wasn't the one who thinks this first: https://rust-lang.github.io/rfcs/2383-lint-reasons.html