Auto merge of #126326 - eggyal:ununsafe-StableOrd, r=michaelwoerister

Un-unsafe the `StableOrd` trait

Whilst incorrect implementations of this trait can cause miscompilation, they cannot cause memory unsafety in rustc.

[Discussed on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/Policy.20of.20.60unsafe.60.20within.20the.20compiler).

cc [MCP 533](rust-lang/compiler-team#533), #105175, `@michaelwoerister`

r? `@Nilstrieb`

Author: bors