Split rustc_mir

The `rustc_mir` crate is the second largest in the compiler.
This PR splits it up into 5 crates:
- rustc_borrowck;
- rustc_const_eval;
- rustc_mir_dataflow;
- rustc_mir_transform;
- rustc_monomorphize.

Rollup of 9 pull requests

Successful merges:

 - #86263 (Rustdoc: Report Layout of enum variants)
 - #88541 (Add regression test for #74400)
 - #88553 (Improve diagnostics for unary plus operators (#88276))
 - #88594 (More symbolic doc aliases)
 - #88648 (Correct “copies” to “moves” in `<Option<T> as From<T>>::from` doc, and other copyediting)
 - #88691 (Add a regression test for #88649)
 - #88694 (Drop 1.56 stabilizations from 1.55 release notes)
 - #88712 (Fix docs for `uX::checked_next_multiple_of`)
 - #88726 (Fix typo in `const_generics` replaced with `adt_const_params` note)

Failed merges:

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

Fix typo in `const_generics` replaced with `adt_const_params` note

Fix docs for `uX::checked_next_multiple_of`

Thanks to `@photino` for noticing this [here](https://github.com/rust-lang/rust/issues/88581#issuecomment-913982246).

r? `@joshtriplett`

`@rustbot` label: +A-docs +A-waiting-on-review

Drop 1.56 stabilizations from 1.55 release notes

r? `@Mark-Simulacrum`

Add a regression test for #88649

I noticed that #88649 does not have a regression test, so I add one in this PR.

The test fails with this without #88678:
```
error[E0080]: evaluation of constant value failed
  --> /checkout/src/test/ui/consts/issue-88649.rs:13:52
   |
LL |             Foo::Variant1(x) | Foo::Variant2(x) if x => {}
   |                                                    ^ StorageLive on a local that was already live

error: aborting due to previous error

For more information about this error, try `rustc --explain E0080`.
```

Correct “copies” to “moves” in `<Option<T> as From<T>>::from` doc, and other copyediting

The `impl<T> From<T> for Option<T>` has no `Copy` or `Clone` bound, so its operation is guaranteed to be a move. The call site might copy, but the function itself cannot.

Since that would have been a rather small PR, I also reviewed the other documentation in the file and made other improvements (in separate commits): adding periods and commas, linking `Deref::Target`, and clarifying what "a container" is in `FromIterator`.

More symbolic doc aliases

A bunch of small changes, mostly adding `#[doc(alias = "…")]` entries for symbolic `"…"`.

Also a small change in documentation of `const` keywords.

Improve diagnostics for unary plus operators (#88276)

This pull request improves the diagnostics emitted on parsing a unary plus operator. See #88276.

Before:

```
error: expected expression, found `+`
 --> src/main.rs:2:13
  |
2 |     let x = +1;
  |             ^ expected expression
```

After:

```
error: leading `+` is not supported
 --> main.rs:2:13
  |
2 |     let x = +1;
  |             ^
  |             |
  |             unexpected `+`
  |             help: try removing the `+`
```

Add regression test for #74400

Closes #74400 by adding a regression test.

Rustdoc: Report Layout of enum variants

Followup of #83501, Fixes #86253.

cc `@camelid`

`@rustbot` label A-rustdoc

`rustdoc`: compute correct line number for indented rust code blocks.

This PR fixes a bug in `rustdoc` where it computes the wrong line number for indented rust code blocks (and subsequent blocks) it finds in markdown strings. To fix this issue, we decrement the line number if we find characters between the code block and the preceding line ending. I noticed this issue as I was trying to use `rustdoc` to extract examples from The Rust Reference and run them through the [Rust Model Checker](https://github.com/model-checking/rmc).

Suggest deriving traits if possible

This only applies to builtin derives as I don't think there is a
clean way to get the available derives in typeck.

Closes #85851

Remove `hir::GenericBound::Unsized`

Rather than "moving" the `?Sized` bounds to the param bounds, just also check where clauses in `astconv`. I also did some related cleanup here, but that's not strictly neccesary. Also going to do a perf run here.

r? `@estebank`

2229: Don't move out of drop type

Fixes #88476

r? `@nikomatsakis`

Bump RLS submodule

r? `@Mark-Simulacrum`