Refactor AST trait bound modifiers
Instead of having two types to represent trait bound modifiers in the parser / the AST (`parser::ty::BoundModifiers` & `ast::TraitBoundModifier`), only to map one to the other later, just use `parser::ty::BoundModifiers` (moved & renamed to `ast::TraitBoundModifiers`).
The struct type is more extensible and easier to deal with (see [here](https://github.com/rust-lang/rust/pull/119099/files#r1430749981) and [here](https://github.com/rust-lang/rust/pull/119099/files#r1430752116) for context) since it more closely models what it represents: A compound of two kinds of modifiers, constness and polarity. Modeling this as an enum (the now removed `ast::TraitBoundModifier`) meant one had to add a new variant per *combination* of modifier kind, which simply isn't scalable and which lead to a lot of explicit non-DRY matches.
NB: `hir::TraitBoundModifier` being an enum is fine since HIR doesn't need to worry representing invalid modifier kind combinations as those get rejected during AST validation thereby immensely cutting down the number of possibilities.
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