Introduce ConstAllocation.

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.

Author: nnethercote

clippy_utils/src/consts.rs

@@ -593,7 +593,7 @@ pub fn miri_to_const(result: ty::Const<'_>) -> Option<Constant> {
         ty::ConstKind::Value(ConstValue::Slice { data, start, end }) => match result.ty().kind() {
             ty::Ref(_, tam, _) => match tam.kind() {
                 ty::Str => String::from_utf8(
-                    data.inspect_with_uninit_and_ptr_outside_interpreter(start..end)
+                    data.inner().inspect_with_uninit_and_ptr_outside_interpreter(start..end)
                         .to_owned(),
                 )
                 .ok()
@@ -605,7 +605,7 @@ pub fn miri_to_const(result: ty::Const<'_>) -> Option<Constant> {
         ty::ConstKind::Value(ConstValue::ByRef { alloc, offset: _ }) => match result.ty().kind() {
             ty::Array(sub_type, len) => match sub_type.kind() {
                 ty::Float(FloatTy::F32) => match miri_to_const(*len) {
-                    Some(Constant::Int(len)) => alloc
+                    Some(Constant::Int(len)) => alloc.inner()
                         .inspect_with_uninit_and_ptr_outside_interpreter(0..(4 * len as usize))
                         .to_owned()
                         .chunks(4)
@@ -619,7 +619,7 @@ pub fn miri_to_const(result: ty::Const<'_>) -> Option<Constant> {
                     _ => None,
                 },
                 ty::Float(FloatTy::F64) => match miri_to_const(*len) {
-                    Some(Constant::Int(len)) => alloc
+                    Some(Constant::Int(len)) => alloc.inner()
                         .inspect_with_uninit_and_ptr_outside_interpreter(0..(8 * len as usize))
                         .to_owned()
                         .chunks(8)