Rollup merge of #118833 - Urgau:lint_function_pointer_comparisons, r=cjgillot

Add lint against function pointer comparisons

This is kind of a follow-up to rust-lang/rust#117758 where we added a lint against wide pointer comparisons for being ambiguous and unreliable; well function pointer comparisons are also unreliable. We should IMO follow a similar logic and warn people about it.

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## `unpredictable_function_pointer_comparisons`

*warn-by-default*

The `unpredictable_function_pointer_comparisons` lint checks comparison of function pointer as the operands.

### Example

```rust
fn foo() {}
let a = foo as fn();

let _ = a == foo;
```

### Explanation

Function pointers comparisons do not produce meaningful result since they are never guaranteed to be unique and could vary between different code generation units. Furthermore different function could have the same address after being merged together.

----

This PR also uplift the very similar `clippy::fn_address_comparisons` lint, which only linted on if one of the operand was an `ty::FnDef` while this PR lints proposes to lint on all `ty::FnPtr` and `ty::FnDef`.

```@rustbot``` labels +I-lang-nominated

~~Edit: Blocked on rust-lang/libs-team#323 being accepted and it's follow-up pr~~

Author: fmease

Diff for: tests/pass/function_pointers.rs

@@ -1,3 +1,5 @@
+#![allow(unpredictable_function_pointer_comparisons)]
+
 use std::mem;
 
 trait Answer {

Diff for: tests/pass/issues/issue-91636.rs

@@ -1,3 +1,5 @@
+#![allow(unpredictable_function_pointer_comparisons)]
+
 type BuiltIn = for<'a> fn(&str);
 
 struct Function {