simplify Target::GenericParam

[mejrs]

Primary motivation is that i've been greatly annoyed recently by us having 3 enums all named GenericParamKind.

Small change in this PR: generic type parameters can now be marked stable/unstable without having a default, i.e.

struct X<#[unstable(feature = "name", reason = "...", issue = "1337")] T>{t: T}

but this is highly unstable so I don't think it matters

[rustbot]

Some changes occurred in compiler/rustc_passes/src/check_attr.rs

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred in compiler/rustc_attr_ir

cc @jdonszelmann, @JonathanBrouwer

[rustbot]

r? @JohnTitor

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