Remove AnonConstKind::GCA and reject generic anon consts

[LaneAsade]

Previously, a GCA type const (type const FOO<const N: usize>: usize = const { N + 1 };) was a type-system-transparent type const whose body immediately thunked out to an opaque regular const (albeit an anon const). The fact that there was a transparent wrapper wasn't providing anything of value, now that the user can write and #155341.

With this PR, GCA variant is removed from AnonConstKind so that generic anon consts are rejected.

Related: rust-lang/project-const-generics#113

[rustbot]

HIR ty lowering was modified

cc @fmease

[rustbot]

r? @chenyukang

rustbot has assigned @chenyukang.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler, types
• compiler, types expanded to 73 candidates
• Random selection from 19 candidates

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[LaneAsade]

I had to open a new PR as the other one closed for some reason and I am not able to reopen it. Sorry 😭

[BoxyUwU]

r? BoxyUwU

[BoxyUwU]

dont worry about needing to open the second PR 😅 what happened is github doesn't let you re-open closed PRs if they've been pushed to since closing :3