Auto merge of #51246 - vakaras:select_polonius_algo_via_env_var, r=nikomatsakis

Auto merge of #51246 - vakaras:select_polonius_algo_via_env_var, r=nikomatsakis

Select Polonius algorithm via `POLONIUS_ALGORITHM` environment variable

This pull request allows selecting the Polonius algorithm being used by providing an environment variable `POLONIUS_ALGORITHM`.

Example usage:
```
POLONIUS_ALGORITHM=compare RUST_LOG=rustc_mir::borrow_check::nll=trace  ./x.py test --stage 1 --compare-mode polonius -- src/test/ui/nll/issue-47680.rs
...
stderr:
------------------------------------------
 INFO 2018-05-31T17:35:31Z: rustc_mir::borrow_check::nll: Using Polonius algorithm: Compare
 INFO 2018-05-31T17:35:31Z: rustc_mir::borrow_check::nll: Using Polonius algorithm: Compare

------------------------------------------
...
```

r? @nikomatsakis