8019fa0dc0
Initial work on Miri permissive-exposed-provenance Rustc portion of the changes for portions of a permissive ptr-to-int model for Miri. The main changes here are changing `ptr_get_alloc` and `get_alloc_id` to return an Option, and also making ptr-to-int casts have an expose side effect.
For more information about how rustc works, see the rustc dev guide.