Identifiers beginning with '__' are not required to be snake case

[jonathanbuchanan]

This change fixes #32653. As suggested by @pnkfelix, a snake case check will return true when the string begins with '__', even if it is not snake case.

This code normally produces an error:
format!("foo {N}", N=name)

This happens because the code generates a variable named __argN, which does not comply with the snake case lint. However, under the patch, that would not cause the lint to throw a warning because it begins with two underscores.

[rust-highfive]

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[frewsxcv]

Related: #19475

[alexcrichton]

Thanks for the PR! Modifications of lints, however, are pretty serious at this stage of the game so this likely needs to go through an RFC-like review at this point. I'd encourage you to start a discussion on our internals forum first if you'd like to still pursue this change.