Auto merge of #155258 - ShoyuVanilla:eq-constraint, r=lcnr

Make region equality emits Eq constraints



For context, see..

- The box named *coroutine witness Send lifetime requirements now considered by leakcheck* from [this roadmap](https://raw.githubusercontent.com/hexcatnl/roadmap/6f23e638f65249ef02af86a5454275103a71552d/next-solver.svg)
- [#t-types/trait-system-refactor > A question on #251 @ 💬](https://rust-lang.zulipchat.com/#narrow/channel/364551-t-types.2Ftrait-system-refactor/topic/A.20question.20on.20.23251/near/584166935)
- Comments on `rustc_type_ir::predicate::RegionEqPredicate`