Support type-relative assoc item paths in generic param defaults & const param types

[fmease]

Background info: For resolving type-relative associated item paths, we only support a tiny set of self types (cc #22519). If it's a non-Self type parameter we look at the "type param bounds" of the overarching item.

However, in the case of generic parameter defaults and const parameter types this overarching "item" was set to the generic parameter. This meant that in these places we didn't look at the bounds of the overarching item when trying to resolve such type-relative associated item paths and thus failed unconditionally.

Fixes #87682.

For the types FCP: This PR makes us accept the following code (stably):

trait Trait { type Type; }

struct Owner<T: Trait, U = T::Type>(T, U);
//                         ~~~~~~~ now successfully resolves to <T as Trait>::Type

As alluded to, this PR also affects the resolution in const param defaults+types (intentionally, of course). However, I don't think this can be observed without the use of unstable features (like min_generic_const_args and generic_const_parameter_types). Nonetheless if you're interested in that, too, please check out the added UI tests.

To the best of my knowledge this change doesn't make us reject more code (via new ambiguities or query cycles for example).

_{(No LLM was or will be used by me during the entire creation process of this PR)}

[fmease]

r? types

[fmease]

@rfcbot merge types

[rust-rfcbot]

@fmease has proposed to merge this. The next step is review by the rest of the tagged team members:

• @BoxyUwU
• @jackh726
• @lcnr
• @oli-obk
• @spastorino

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

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[fmease]

Not sure if this needs an FCP but it felt "significant" enough. Since there's no breakage AFAIK & it's a bit niche, this might also be considered a trivial bugfix in which case I'm sorry for the team ping ^^'