Support pointers in type reflection

[BD103]

Tracking issue: #146922

This PR adds support for inspecting pointers *const T and *mut T through type reflection. It does so by adding the new Pointer struct + variant:

pub struct Pointer {
    /// The type of the value being pointed to.
    pub ty: TypeId,
    /// Whether this pointer is mutable or not.
    pub mutable: bool,
}

This can be gathered using Type::of, for example:

match const { Type::of::<*const u8>() }.kind {
    TypeKind::Pointer(pointer) => {
        assert_eq!(pointer.ty, TypeId::of::<u8>());
        assert!(!pointer.mutable);
    }
    _ => unreachable!(),
}

[rustbot]

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

The reflection data structures are tied exactly to the implementation
in the compiler. Make sure to also adjust rustc_const_eval/src/const_eval/type_info.rs

cc @oli-obk

[rustbot]

r? @JonathanBrouwer

rustbot has assigned @JonathanBrouwer.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[BD103]

r? @oli-obk

[rustbot]

oli-obk is not on the review rotation at the moment.
They may take a while to respond.

[rust-bors]

☔ The latest upstream changes (presumably #151210) made this pull request unmergeable. Please resolve the merge conflicts.