Add global_variables lint

[EFanZh]

changelog: [global_variables]: check static variables with interior mutability.

[rustbot]

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[Jarcho]

This looks like it's the same lint as declare_interior_mutable_const but for static items. For consistency this should be named declare_interior_mutable_statics (plural, the other one violates the naming guide) and should share the same implementation.

[EFanZh]

@Jarcho: I have moved the implementation together with declare_interior_mutable_const, but I didn’t change the name because global_variables seems to be easier to understand than declare_interior_mutable_statics. If you still prefer your proposed name, I am OK to change it.

Also, I am not able to exclude statics with interior mutability but with “frozen” initializers. For example: A static using None::<AtomicU32> as the initializer can be excluded. This detection used for consts uses the is_value_unfrozen_poly function, which panics for statics. What should I do from here?

• Ignore this false positive, since using a interior mutable static that isn’t actually mutable does not make much sense, the impact could be minor, or
• Continue trying to figure out how to detect the case with non-mutable value case.

[rustbot]

☔ The latest upstream changes (possibly 94cfebb) made this pull request unmergeable. Please resolve the merge conflicts.