Mark `MutexGuard` with `#[clippy::has_significant_drop]`

[c410-f3r]

#[clippy::has_significant_drop] tells that a structure should be considered when evaluating some lints. Examples of such behavior are the existent clippy::significant_drop_in_scrutinee and the soon-to-be-finished https://github.com/rust-lang/rust-clippy/issues/9399.

[mkroening]

@c410-f3r, should we also apply this to all the other guards, like the RwLock guards and the reentrant mutex guard, as well as their mapped variants?

[c410-f3r]

Yes, any lock primitive should apply #[clippy::has_significant_drop] in order to trigger the two mentioned lints.

Moreover, it is very likely that other lints will also use #[clippy::has_significant_drop] in the future.

[mkroening]

I opened https://github.com/Amanieu/parking_lot/pull/371.

[c410-f3r]

Nice! Thank you