Implement a MIR parser for testing purposes

[fee1-dead]

As discussed in Zulip (archive), we'd want to be able to write tests where the input is MIR and not Rust to test soundness of certain MIR optimizations. This would require a MIR parser for internal usage only and test mir-opt with that. Codegen tests would probably benefit from this as well.

See also rust-lang/miri#196

[spunit262]

@rustbot claim

[fee1-dead]

@spunit262 thank you for being willing to tackle this issue. If you get stuck, you can ask a question here by posting a comment or post a message on zulip. You can also dm anyone from @rust-lang/wg-const-eval (me, @oli-obk, @RalfJung, @lcnr), we'd be happy to help out.

[oli-obk]

we now have #[custom_mir] that, while still having a few missing pieces to implement, is exactly implementing what this issue is requesting