Auto merge of #78429 - casey:doctest-attribute-splitting, r=jyn514

[librustdoc] Only split lang string on `,`, ` `, and `\t`

Split markdown lang strings into tokens on `,`.

The previous behavior was to split lang strings into tokens on any
character that wasn't a `_`, `_`, or alphanumeric.

This is a potentially breaking change, so please scrutinize! See discussion in #78344.

I noticed some test cases that made me wonder if there might have been some reason for the original behavior:

```
t("{.no_run .example}", false, true, Ignore::None, true, false, false, false, v(), None);
t("{.sh .should_panic}", true, false, Ignore::None, false, false, false, false, v(), None);
t("{.example .rust}", false, false, Ignore::None, true, false, false, false, v(), None);
t("{.test_harness .rust}", false, false, Ignore::None, true, true, false, false, v(), None);
```

It seemed pretty peculiar to specifically test lang strings in braces, with all the tokens prefixed by `.`.

I did some digging, and it looks like the test cases were added way back in [this commit from 2014](https://github.com/rust-lang/rust/commit/3fef7a74ca9a) by `@skade.`

It looks like they were added just to make sure that the splitting was permissive, and aren't testing that those strings in particular are accepted.

Closes https://github.com/rust-lang/rust/issues/78344.

compiler/rustc_mir/src/dataflow/framework/mod.rs

@@ -10,7 +10,7 @@
 //! fixpoint solution to your dataflow problem, or implement the `ResultsVisitor` interface and use
 //! `visit_results`. The following example uses the `ResultsCursor` approach.
 //!
-//! ```ignore(cross-crate-imports)
+//! ```ignore (cross-crate-imports)
 //! use rustc_mir::dataflow::Analysis; // Makes `into_engine` available.
 //!
 //! fn do_my_analysis(tcx: TyCtxt<'tcx>, body: &mir::Body<'tcx>) {
@@ -211,7 +211,7 @@ pub trait Analysis<'tcx>: AnalysisDomain<'tcx> {
     /// default impl and the one for all `A: GenKillAnalysis` will do the right thing.
     /// Its purpose is to enable method chaining like so:
     ///
-    /// ```ignore(cross-crate-imports)
+    /// ```ignore (cross-crate-imports)
     /// let results = MyAnalysis::new(tcx, body)
     ///     .into_engine(tcx, body, def_id)
     ///     .iterate_to_fixpoint()