Auto merge of #49861 - pnkfelix:compare-mode-nll-followup-2, r=nikomatsakis

Auto merge of #49861 - pnkfelix:compare-mode-nll-followup-2, r=nikomatsakis

Blindly checkpoint status of NLL mode ui tests

This takes the next (and potentially final?) step with #48879.

Namely, this PR got things to the point where I can successfully run `compiletest` on `src/test/ui` with `--compile-mode=nll`.

Here are the main pieces of it:

 1. To figure out how to even run `compiletest` normally on the ui directory, I ran `x.py test -vv`, and then looked for the `compiletest` invocation that mentioned `src/test/ui`.
 2. I took the aforementioned `compiletest` invocation and used it, adding `--compile-mode=nll` to the end. It had 170 failing cases.
 3. Due to #49855, I had to edit some of the tests so that they fail even under NLL, via `#[rustc_error]`. That's the first commit. (Then goto 2 to double-check no such tests remain.)
 4. I took the generated `build/target/test/foo.stderr` file for every case that failed, and blindly copied it to `src/test/foo.nll.stderr`. That's the second commit.
 5. Goto 2 until there were no failing cases.
 6. Remove any stamp files, and re-run `x.py test` to make sure that the edits and new `.nll.stderr` files haven't broken the pre-existing test suite.