Currently some `Allocation`s are interned, some are not, and it's very
hard to tell at a use point which is which.

This commit introduces `ConstAllocation` for the known-interned ones,
which makes the division much clearer. `ConstAllocation::inner()` is
used to get the underlying `Allocation`.

In some places it's natural to use an `Allocation`, in some it's natural
to use a `ConstAllocation`, and in some places there's no clear choice.
I've tried to make things look as nice as possible, while generally
favouring `ConstAllocation`, which is the type that embodies more
information. This does require quite a few calls to `inner()`.

The commit also tweaks how `PartialOrd` works for `Interned`. The
previous code was too clever by half, building on `T: Ord` to make the
code shorter. That caused problems with deriving `PartialOrd` and `Ord`
for `ConstAllocation`, so I changed it to build on `T: PartialOrd`,
which is slightly more verbose but much more standard and avoided the
problems.

`Layout` is another type that is sometimes interned, sometimes not, and
we always use references to refer to it so we can't take any advantage
of the uniqueness properties for hashing or equality checks.

This commit renames `Layout` as `LayoutS`, and then introduces a new
`Layout` that is a newtype around an `Interned<LayoutS>`. It also
interns more layouts than before. Previously layouts within layouts
(via the `variants` field) were never interned, but now they are. Hence
the lifetime on the new `Layout` type.

Unlike other interned types, these ones are in `rustc_target` instead of
`rustc_middle`. This reflects the existing structure of the code, which
does layout-specific stuff in `rustc_target` while `TyAndLayout` is
generic over the `Ty`, allowing the type-specific stuff to occur in
`rustc_middle`.

The commit also adds a `HashStable` impl for `Interned`, which was
needed. It hashes the contents, unlike the `Hash` impl which hashes the
pointer.

This fixes a miscompilation

This commit makes `AdtDef` use `Interned`. Much the commit is tedious
changes to introduce getter functions. The interesting changes are in
`compiler/rustc_middle/src/ty/adt.rs`.

This reduces the amount of unstable features used by cg_clif

Only rustc_private is still enabled as cg_clif by definition needs to
use internal rustc api's.

This reduces binary sizes by a decent amount:

libstd.so: 17% reduction
mini_core_hello_world: 27% reduction
simple-raytracer: 27% reduction

This also improves compile time of simple-raytracer by 0.5s (4% +- 2%)
In addition it is also a pre-requisite for building standalone binaries.

Replace a lot of print+trap with plain trap

This saves 40-50s on CI as the repo history can be skipped

Run more rustc tests

Part of rust-lang#806

128bit atomics are unstable and only enabled on AArch64 and x86_64 macOS.
Cranelift doesn't support 128bit atomics yet.

…clif-2022-03-20