make never_loop applicability more flexible

[lapla-cogito]

The applicability of never_loop is currently set to Unspecified, but if the loop block does not contain break or continue, it can be MachineApplicable.

changelog: [never_loop]: the applicability is now MachineApplicable when the loop block contains neither break nor continue

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[samueltardieu]

You should create a tests/ui/never_loop_fixable.rs to test the fixed version, which would not include //@no-rustfix (which never_loop.rs includes).

[lapla-cogito]

Thanks, I've ported the test for this patch to a new file.

[lapla-cogito]

Rebased to address the recent changes in error annotation.