Annotate SMIR with lifetime information

The rust compiler intentionally hides all lifetime information to avoid introducing accidental dependencies on it during compilation and to improve the effectiveness of caching throughout the compiler. Tools like Prusti, Verus or Gillian-Rust on the other hand depend on having access to precise information about the borrows and lifetimes available in a given function.

Currently, a compromise solution has been reached with the compiler giving partial visibility into some of the information calculated by borrowck, allowing tool authors to mostly achieve their goals, though often with significant work on their end.

If SMIR included lifetime information, this would be a killer feature, providing something of huge value to external users while also allowing rustc keep its borrowck implementation private.

Specifically, I would like borrows to be annotated with their regions, and for those regions to have been unified throughout the MIR body. It should also be easy to regions are universal, and to answer outlives questions between regions.

The issue does not name files, tests, or entry points. Start by reviewing SMIR's current visibility into borrowck information and the requirements for annotating borrows with unified regions, identifying universal regions, and answering outlives questions. Done means SMIR exposes this lifetime information without exposing rustc's borrowck implementation.