Update mgca to use `type const` syntax instead of the `#[type_const]` attribute. 

This PR changes the `#[type_const]` attribute to the `type const` syntax for  #132980.

This will fixes #151273 and similar issues, since we need to check `type const` of items before expansion. The move to add a syntax was mentioned here: #151289 (comment)

The first part of this PR adds support by allowing `type const <IDENT>: <TYPE> { = <EXPR> };` syntax in `rustc_parse/src/parser/item.rs`.

The next part since the AST item does not contain enough information to determine if we have a `type const` was rework `ConstItemRhs` into `ConstItemRhsKind` to store the information since we no longer have the attribute acting as a source of extra data/metadata. 

The hir node `ConstItemRhsKind` current shape mostly works, except in the case of `TraitItemKind` where it is an option. I initially went about giving `hir::ConstItemRhsKind` a similar form the AST, but it touches a lot more lines of code and files so because of that, the less invasive option was to add a simple boolean flag to `TraitItemKind`. 

The forth part of this PR includes adding a query I called `is_rhs_type_const` so that we can handle both local and foreign def_ids. 

The fifth aspect of the PR is adding a `mgca_type_const_syntax` feature gate that is checked before expansion. The standard mgca feature gate is ran after expansion. This feature gate allows for conditional compilation (e.g #[cfg(..)]) of the `type const` syntax  in nightly without `min_generic_const_args` being enabled. 

The last bit is updating all the the tests that used the `#[type_const]` attribute to use the new syntax that failed because of the changes. This is the bulk of touched/edited files in the PR. 

r? @BoxyUwU 
@rustbot label +F-associated_const_equality +F-min_generic_const_args