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.

compiler/rustc_const_eval/src/interpret/machine.rs

@@ -13,8 +13,9 @@ use rustc_target::abi::Size;
 use rustc_target::spec::abi::Abi;
 
 use super::{
-    AllocId, AllocRange, Allocation, Frame, ImmTy, InterpCx, InterpResult, LocalValue, MemPlace,
-    Memory, MemoryKind, OpTy, Operand, PlaceTy, Pointer, Provenance, Scalar, StackPopUnwind,
+    AllocId, AllocRange, Allocation, ConstAllocation, Frame, ImmTy, InterpCx, InterpResult,
+    LocalValue, MemPlace, Memory, MemoryKind, OpTy, Operand, PlaceTy, Pointer, Provenance, Scalar,
+    StackPopUnwind,
 };
 
 /// Data returned by Machine::stack_pop,
@@ -252,7 +253,7 @@ pub trait Machine<'mir, 'tcx>: Sized {
     fn before_access_global(
         _memory_extra: &Self::MemoryExtra,
         _alloc_id: AllocId,
-        _allocation: &Allocation,
+        _allocation: ConstAllocation<'tcx>,
         _static_def_id: Option<DefId>,
         _is_write: bool,
     ) -> InterpResult<'tcx> {