Merge `IdxSet` and `IdxSetBuf`

Because it simplifies things.

@r? nikomatsakis

Allow panicking with string literal messages inside constants

r? @EddyB

cc https://github.com/rust-lang/rust/issues/51999

we can't implement things like `panic!("foo: {}", x)` right now because we can't call trait methods (most notably `Display::fmt`) inside constants. Also most of these impls probably have loops and conditions, so it's messy anyway.

But hey `panic!("foo")` works at least.

cc @japaric got any test ideas for `#![no_std]`?

Rollup of 10 pull requests

Successful merges:

 - #53418 (Mark some suggestions as MachineApplicable)
 - #53431 (Moved some feature gate ui tests to correct location)
 - #53442 (Update version of rls-data used with save-analysis)
 - #53504 (Set applicability for more suggestions.)
 - #53541 (Fix missing impl trait display as ret type)
 - #53544 (Point at the trait argument when using unboxed closure)
 - #53558 (Normalize source line and column numbers.)
 - #53562 (Lament the invincibility of the Turbofish)
 - #53574 (Suggest direct raw-pointer dereference)
 - #53585 (Remove super old comment on function that parses items)

Failed merges:

 - #53472 (Use FxHash{Map,Set} instead of the default Hash{Map,Set} everywhere in rustc.)
 - #53563 (use String::new() instead of String::from(""), "".to_string(), "".to_owned() or "".into())

r? @ghost

Rename TyVariants and variants

- Rename `TypeVariants` to `TyKind`.
- Remove the `Ty` prefix from each one of its variants (plus the identically-named variants of `PrimTy`).
- Rename `ty::Slice` to `ty::List`.

The new names look cleaner.

r? @EddyB

Remove super old comment on function that parses items

This comment was added more than 5 years ago in ab03c1e4. As far as anyone reading this comment today needs to know, the function has never parsed items from inside an extern crate.

Suggest direct raw-pointer dereference

People often come looking for some kind of `as_ref_unchecked` method on
raw pointers that would give them `&T` and not `Option<&T>` when they
are sure the pointer is not NULL.

There's no such method, but taking a reference of the dereferenced
pointer accomplishes the same thing. Therefore, suggest using that, at
the `as_ref` site ‒ it's a place people are likely going to look into.

Lament the invincibility of the Turbofish

Here a test case is added to ensure that any others attempting to drive the Turbofish to extinction have second thoughts. Previously the [entire test suite would succeed](https://github.com/rust-lang/rust/pull/53511) if generic arguments were accepted without disambiguation, making for [confusing and heartbreaking circumstances](https://github.com/rust-lang/rfcs/pull/2527).

Normalize source line and column numbers.

Fixes #53547.

r? @EddyB

Point at the trait argument when using unboxed closure

Fix #53534.

r? @varkor

Fix missing impl trait display as ret type

I need to convert a `TraitPredicate` into a `TraitBound` to get the returned impl trait. So far, didn't find how or even if it was the good way to do it.

cc @EddyB @oli-obk (since you're the one behind the change apparently 😉)

Set applicability for more suggestions.

Converts a couple more calls to `span_suggestion_with_applicability`  (#50723). To be on the safe side, I marked suggestions that depend on the intent of the user or that are potentially lossy conversions as MaybeIncorrect.

r? @estebank

Update version of rls-data used with save-analysis

This part 1/3 for fixing rust-lang/rust#53440.

Moved some feature gate ui tests to correct location

None

Mark some suggestions as MachineApplicable

I think the following suggestions should be safe to mark as `MachineApplicable`.

r? @estebank

fix array drop glue: properly turn raw ptr into reference

Discovered while working on https://github.com/rust-lang/rust/pull/53424: The generated drop glue uses an assignment `ptr = cur` where `ptr` is a reference and `cur` a raw pointer. This is not well-formed MIR.

Do we have MIR sanity checks that run on the drop glue and should have caught this?

r? @EddyB

Remove Ty prefix from Ty{Adt|Array|Slice|RawPtr|Ref|FnDef|FnPtr|Dynamic|Closure|Generator|GeneratorWitness|Never|Tuple|Projection|Anon|Infer|Error}

Buffer LLVM's object output stream

In some profiling on OSX I saw the `write` syscall as quite high up on
the profiling graph, which is definitely not good! It looks like we're
setting the output stream of an object file as directly to a file
descriptor which means that we run the risk of doing lots of little
writes rather than a few large writes.

This commit fixes this issue by adding a buffered stream on the output,
causing the `write` syscall to disappear from the profiles on OSX.

CTFE engine refactor

* Value gets renamed to `Operand`, so that now `interpret::{Place, Operand}` are the "dynamic" versions of `mir::{Place, Operand}`.
* `Operand` and `Place` share the data for their "stuff is in memory"-base in a new type, `MemPlace`. This also makes it possible to give some more precise types in other areas. Both `Operand` and `MemPlace` have methods available to project into fields (and other kinds of projections) without causing further allocations.
* The type for "a `Scalar` or a `ScalarPair`" is called `Value`, and again used to give some more precise types.
* All of these have versions with an attached layout, so that we can more often drag the layout along instead of recomputing it. This lets us get rid of `PlaceExtra::Downcast`. `MPlaceTy` and `PlaceTy` can only be constructed in place.rs, making sure the layout is handled properly. (The same should eventually be done for `ValTy` and `OpTy`.)
 This is used to check, when copying an operand to a place, that the sizes match (which caught a bunch of bugs).
* All the high-level functions to write typed memory take a `Place`, and live in `place.rs`. All the high-level typed functions to read typed memory take an `Operand`, and live in `operands.rs`.
* Remove `cur_frame` and handling of signedess from memory (catching a bug in the float casting code).
* [Only functional change] Enable sanity check to recurse below dyn traits and slices.

r? @oli-obk

Cc @EddyB

During the sanity check, we keep track of the path we are below in a `Vec`.  We
avoid cloning that `Vec` unless we hit a pointer indirection.  The `String`
representation is only computed when validation actually fails.