Split inline const to two feature gates and mark expression position inline const complete

This PR splits inline const in pattern position into its own `#![feature(inline_const_pat)]` feature gate, and make the usage in expression position complete.

I think I have resolved most outstanding issues related to `inline_const` with rust-lang#89561 and other PRs. The only thing left that I am aware of is rust-lang#90150 and the lack of lifetime checks when inline const is used in pattern position (FIXME in rust-lang#89561). Implementation-wise when used in pattern position it has to be lowered during MIR building while in expression position it's evaluated only when monomorphizing (just like normal consts), so it makes some sense to separate it into two feature gates so one can progress without being blocked by another.

`@rustbot` label: T-compiler F-inline_const

…askrgr

Rollup of 6 pull requests

Successful merges:

 - rust-lang#90856 (Suggestion to wrap inner types using 'allocator_api' in tuple)
 - rust-lang#91103 (Inhibit clicks on summary's children)
 - rust-lang#91137 (Give people a single link they can click in the contributing guide)
 - rust-lang#91140 (Split inline const to two feature gates and mark expression position inline const complete)
 - rust-lang#91148 (Use `derive_default_enum` in the compiler)
 - rust-lang#91153 (kernel_copy: avoid panic on unexpected OS error)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup