Move unused-attribute pass after trans

[sfackler]

Many attributes are not used until after trans, but the current lint pass runs before it. Those attributes are currently globally whitelisted, but this isn't a great long term solution.

[emberian]

@sfackler this would require keeping the AST and all analysis data through trans, right?

[sfackler]

That or performing the check as trans consumes the AST.

[jonas-schievink]

Looks like this could cause issues with -Zno-trans

[Mark-Simulacrum]

I suspect that this is superseded by #17642. If you'd like to reopen with a specific set of attributes that should be handled, then we can do so (for example, #[used] is now special-cased: #41637).