Auto merge of #17514 - HarveyHunt:deny-public-fields-error-message, r=jdm

deny_public_fields: display field identifier in help message

Use a binding's field identifier to produce more descriptive output.
Fall back to a binding's identifier if a field identifier is not
available.

Additionally, make the privatize.rs compiletest more specific and
wrap the identifier in backticks to aid readability.

As an aside, should I send a PR to rename ```compile-fail/privatize.rs``` to ```compile-fail/deny_public_fields.rs``` as privatize isn't mentioned in the codebase any more?

```
$ find . -name "*privatize*"
./tests/compiletest/plugin/compile-fail/privatize.rs
```

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