Lint against misplaced where-clauses on associated types in traits

[fmease]

Extends the scope of the lint deprecated_where_clause_location (#89122) from associated types in impls to associated types in any location (impl or trait). This is only relevant for #![feature(associated_type_defaults)]. Previously we didn't warn on the following code for example:

#![feature(associated_type_defaults)]
trait Trait { type Assoc where u32: Copy = (); }

Personally I would've preferred to emit a hard error here instead of a lint warning since the feature is unstable but unfortunately we are constrained by back compat as associated type defaults won't necessarily trigger the feature-gate error if they are inside of a macro call (since they use a post-expansion feature-gate due to historical reasons, see also #66004).

I've renamed and moved related preexisting tests: 1. They test AST validation passes not the parser & thus shouldn't live in parser/ (historical reasons?). 2. One test file was named after type aliases even though it tests assoc tys.

@rustbot label A-lint

[rustbot]

r? @cjgillot

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[compiler-errors]

This makes sense, and given that it only affects nightly feature-gated code and just extends this lint to a position where it was forgotten, I'm gonna approve it.

r? @compiler-errors @bors r+ rollup

[bors]

📌 Commit b809207 has been approved by compiler-errors

It is now in the queue for this repository.