Add DefId::parent() accessor for rustc_public

[jacob-greenfield]

Adds a parent() method to DefId (the rustc_pub version) which exposes the parent path, ie. foo::bar::baz -> foo::bar.

This is useful for organizing/grouping definitions into a tree, and is probably simpler and less brittle than attempting to parse the fully-qualified name into path components (e.g. especially when handling path components with qualified generic parameters).

[rustbot]

r? @scottmcm

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[rustbot]

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a

[makai410]

Thanks! Could you please add an ui-test for that, if it's not too much work

[scottmcm]

Sorry, while I volunteered to help with reviews here I don't really know intent for stuff like this, so re-rolling
r? project-stable-mir

[celinval]

It looks good. Can you please add a test to the new API? You can just augment https://github.com/rust-lang/rust/blob/master/tests/ui-fulldeps/rustc_public/check_defs.rs

r=me when you are done

[celinval]

oops... wrong checkbox.

[celinval]

r? celinval

[Dylan-DPC]

@jacob-greenfield can you add the test required for this? that's the only thing pending here.