Rollup merge of #117745 - ouz-a:emit_smir, r=celinval

Emit smir

This adds ability to `-Zunpretty=smir` and get smir output of a Rust file, this is obliviously pretty basic compared to `mir` output but I think we could iteratively improve it, and even at this state this is useful for us.

r? ``@celinval``

Author: matthiaskrgr