Forbid cannot be overriden. When someome tries to do this anyways,
it results in a hard error. That makes sense.
Except it doesn't, because macros. Macros may reasonably use `#[deny]`
in their expansion to assert
that their expanded code follows the lint. This is doesn't work when the
output gets expanded into a `forbid()` context. This is pretty silly,
since both the macros and the code agree on the lint!
Therefore, we allow `#[deny(..)]`ing a lint that's already forbidden,
keeping the level at forbid.
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