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]`.

Subtask of rust-lang/rust#48879

src/librustc/session/config.rs

@@ -1259,6 +1259,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         useful for profiling / PGO."),
     relro_level: Option<RelroLevel> = (None, parse_relro_level, [TRACKED],
         "choose which RELRO level to use"),
+    nll_subminimal_causes: bool = (false, parse_bool, [UNTRACKED],
+        "when tracking region error causes, accept subminimal results for faster execution."),
     disable_nll_user_type_assert: bool = (false, parse_bool, [UNTRACKED],
         "disable user provided type assertion in NLL"),
     trans_time_graph: bool = (false, parse_bool, [UNTRACKED],