Continuation of #94376.

cc #53667

Specifically, rename the `Const` struct as `ConstS` and re-introduce `Const` as
this:
```
pub struct Const<'tcx>(&'tcx Interned<ConstS>);
```
This now matches `Ty` and `Predicate` more closely, including using
pointer-based `eq` and `hash`.

Notable changes:
- `mk_const` now takes a `ConstS`.
- `Const` was copy, despite being 48 bytes. Now `ConstS` is not, so need a
  we need separate arena for it, because we can't use the `Dropless` one any
  more.
- Many `&'tcx Const<'tcx>`/`&Const<'tcx>` to `Const<'tcx>` changes
- Many `ct.ty` to `ct.ty()` and `ct.val` to `ct.val()` changes.
- Lots of tedious sigil fiddling.

Helps avoid rightward drift.

No functional changes intended.

I'm playing around with building compiler components using nightly rust
(2021-11-02) in a non-standard way. I encountered the following error while
trying to build rustc_mir_build:

```
error[E0597]: `wildcard` does not live long enough
    --> rust/src/nightly/compiler/rustc_mir_build/src/build/matches/mod.rs:1767:82
     |
1767 |         let mut otherwise_candidate = Candidate::new(expr_place_builder.clone(), &wildcard, false);
     |                                                                                  ^^^^^^^^^ borrowed value does not live long enough
...
1799 |     }
     |     -
     |     |
     |     `wildcard` dropped here while still borrowed
     |     borrow might be used here, when `guard_candidate` is dropped and runs the destructor for type `Candidate<'_, '_>`
     |
     = note: values in a scope are dropped in the opposite order they are defined
```

I believe this flags an issue that may become an error in the future.
Swapping the order of `wildcard` and `guard_candidate` resolves it.

Each pattern in a match arm has its own copy of the match guard in MIR,
with its own temporary, so it has to be dropped before the the guards
are joined to the single copy of the arm.

MIR lowering for `if let` expressions is now more complicated now that
`if let` exists in HIR. This PR adds a scope for the variables bound in
an `if let` expression and then uses an approach similar to how we
handle loops to ensure that we reliably drop the correct variables.

Previously, we would set up the source lines for `match` expressions so
that the code generated to perform the test of the scrutinee was matched
to the line of the arm that required the test and then jump from the arm
block to the "next" block was matched to all of the lines in the `match`
expression.

While that makes sense, it has the side effect of causing strange
stepping behavior in debuggers.

I've changed the source information so that all of the generated tests
are sourced to `match {scrutinee}` and the jumps are sourced to the last
line of the block they are inside. This resolves the weird stepping
behavior in all debuggers and resolves some instances of "ambiguous
symbol" errors in WinDbg preventing the user from setting breakpoints at
`match` expressions.

This reverts commit 4fef3911.

Also switches on formatting of the mir build module

The highest-priority item is the *first* in the list, not the last.
See [this code][1] for more.

[this code]: https://github.com/rust-lang/rust/blob/0e63af5da3400ace48a0345117980473fd21ad73/compiler/rustc_mir_build/src/build/matches/mod.rs#L1334-L1335

- Fix typos
- Add more information
- General cleanup

Prior to this commit, debuginfo was always generated by mapping a name
to a Place. This has the side-effect that `SimplifyLocals` cannot remove
locals that are only used for debuginfo because their other uses have
been const-propagated.

To allow these locals to be removed, we now allow debuginfo to point to
a constant value. The `ConstProp` pass detects when debuginfo points to
a local with a known constant value and replaces it with the value. This
allows the later `SimplifyLocals` pass to remove the local.