Fix up a Fluent message

[fmease]

Fix up a Fluent message which contained arrows -> after selectors. The original author probably thought that they were required as part of the selector syntax but in reality they were interpreted as literal text and actually showed up in the emitted diagnostic.

This wasn't caught during the diagnostic migration since the branch constructing the diagnostic in question (rustc_infer::errors::LifetimeMismatchLabels::Normal) was not exercised by the UI test suite. I've added two more test cases to do so (one testing LifetimeMismatchLabels::Normal where hir_equal == true and one where hir_equal == false).

Diff visualizing the -> bug (master vs fix-up-a-fluent-message):

 error[E0623]: lifetime mismatch
   --> src/test/ui/implied-bounds/hrlt-implied-trait-bounds-guard.rs:39:30
    |
 39 | fn badboi3<'in_, 'out, T>(a: Foo<'in_, 'out, (&'in_ T, &'out T)>, sadness: &'in_ T) {
    |                              ^^^^^^^^^^^^^^^^^-------^^-------^^
    |                              |                |
    |                              |                these two types are declared with different lifetimes...
-   |                              ...but data->  from `a` flows->  into `a` here
+   |                              ...but data from `a` flows into `a` here

[rustbot]

r? @compiler-errors

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[rustbot]

rustc_error_messages was changed

cc @davidtwco, @compiler-errors, @JohnTitor, @estebank, @TaKO8Ki

[compiler-errors]

@bors r+ rollup

[bors]

📌 Commit 23dadb5 has been approved by compiler-errors

It is now in the queue for this repository.

[fmease]

Oh no, I forgot to get rid of the -> after the *[false] selectors 🤦‍♂️. I shouldn't be writing PRs at 4:30 AM. Anyway, I am gonna fix that at some point in the future.

[compiler-errors]

🤦 it's ok, it's not like I noticed it either