Auto merge of rust-lang#105356 - JakobDegen:more-custom-mir, r=oli-obk

Custom MIR: Many more improvements

Commits are each atomic changes, best reviewed one at a time, with the exception that the last commit includes all the documentation.

### First commit

Unsafetyck was not correctly disabled before for `dialect = "built"` custom MIR. This is fixed and a regression test is added.

### Second commit

Implements `Discriminant`, `SetDiscriminant`, and `SwitchInt`.

### Third commit

Implements indexing, field, and variant projections.

### Fourth commit

Documents the previous commits and everything else.

There is some amount of weirdness here due to having to beat Rust syntax into cooperating with MIR concepts, but it hopefully should not be too much. All of it is documented.

r? `@oli-obk`

Author: bors