Rollup merge of #144291 - oli-obk:const_trait_alias, r=fee1-dead

Constify trait aliases

Allow `const trait Foo = Bar + [const] Baz;` trait alias declarations. Their rules are the same as with super traits of const traits. So `[const] Baz` or `const Baz` is only required for `[const] Foo` or `const Foo` bounds respectively.

tracking issue rust-lang/rust#41517 (part of the general trait alias feature gate, but I can split it out into a separate const trait alias feature gate. I just assumed that const traits would stabilize before trait aliases, and we'd want to stabilize trait aliases together with const trait aliases at the same time)

r? ``@compiler-errors`` ``@fee1-dead``

Author: matthiaskrgr