Use is_from_proc_macro instead of using snippet

[Gri-ffin]

(Since unsafe blocks can now be linted I'm not sure if I should add a changelog, given this is an internal change)

changelog: none

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[Jarcho]

Why does this need to be a late pass? The todo comment in the old version is just wrong about what the fix should have been.

[Gri-ffin]

I can't speak for the author but I assumed there was a good reasoning behind the comment since it passed the review phase.

[Jarcho]

It looks like it's only about is_from_proc_macro which can just be extended to work on ast::Block.

edit: Just checked and that's why.