Add [extern_without_abi] lint

[CBSpeir]

Fixes #13372

The [extern_without_abi] lint emits when extern is not followed by an ABI. For example:

// EMIT
extern fn foo() {}

// EMIT
extern {
    fn bar();
}

// NO EMIT
extern "C" fn baz() {}

// NO EMIT
extern "C" {
    fn foo_bar();
}

---

changelog: Add [extern_without_abi] lint

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[Centri3]

Overall looks good to me 👍

[bors]

☔ The latest upstream changes (presumably #13322) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #13440) made this pull request unmergeable. Please resolve the merge conflicts.

[CBSpeir]

Thank you for reviewing the PR. It should include the changes you requested. I went ahead and squashed and merged the changes with the latest from master.

[bors]

☔ The latest upstream changes (presumably #13334) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #13395) made this pull request unmergeable. Please resolve the merge conflicts.

[CBSpeir]

In addition to fixing the merge conflicts, I moved is_from_proc_macro test to last since it seems to be the most expensive. Also made a minor change to a string literal to improve readability.

[y21]

There's the missing_abi rustc lint that sounds like the same as what this adds, or is there a difference? rust-lang/rust#132397 makes that rustc lint warn-by-default

[CBSpeir]

I guess missing_abi already checks for this, and therefore this PR isn't needed.

There is one difference, this lint emits a diagnostic with a suggestion. I don't believe missing_abi does.