Auto merge of #49900 - pnkfelix:compare-mode-nll-followup-3, r=nikomatsakis
Add src/test/ui regression testing for NLL
This PR changes `x.py test` so that when you are running the `ui` test suite, it will also always run `compiletest` in the new `--compare-mode=nll`, which just double-checks that when running under the experimental NLL mode, the output matches the `<source-name>.nll.stderr` file, if present.
In order to reduce the chance of a developer revolt in response to this change, this PR also includes some changes to make the `--compare-mode=nll` more user-friendly:
1. It now generates nll-specific .stamp files, and uses them (so that repeated runs can reuse previously cached results).
2. Each line of terminal output distinguishes whether we are running under `--compare-mode=nll` by printing with the prefix `[ui (nll)]` instead of just the prefix `[ui]`.