Compare trait references in trait_duplication_in_bounds correctly

[y21]

Fixes #13476
Fixes #11067
Fixes #9915
Fixes #9626

Currently, the trait_duplication_in_bounds lints has a helper type for a trait reference that can be used for comparison and hashing, represented as {trait: Res, generic_args: Vec<Res>}. However, there are a lot of issues with this. For one, a Res can't represent e.g. references, slices, or lots of other types, as well as const generics and associated type equality. In those cases, the lint simply ignores them and has no way of checking if they're actually the same.

So, instead of using Res for this, use SpanlessEq and SpanlessHash for comparisons with the trait path for checking if there are duplicates.

However, using SpanlessEq as is alone lead to a false negative in the test. std::clone::Clone + foo::Clone wasn't recognized as a duplicate, because it has different segments. So this also adds a new "mode" to SpanlessEq which compares by final resolution. (I've been wondering if this can't just be the default but it's quite a large scale change as it affects a lot of lints and I haven't yet looked at all uses of it to see if there are lints that really do care about having exactly the same path segments).

Maybe an alternative would be to turn the hir types/consts into middle types/consts and compare them instead but I'm not sure there's really a good way to do that

changelog: none

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[Manishearth]

@bors r+

[bors]

📌 Commit d6be597 has been approved by Manishearth

It is now in the queue for this repository.

[bors]

⌛ Testing commit d6be597 with merge a01975b...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Manishearth
Pushing a01975b to master...