refactor: move convert to unstable

We want to keep StableMIR definitions and logic separate from any sort of conversion and usage of internal rustc code. So we bundle all unstable items that have no stability guarantees into `stable_mir::unstable`.

compiler/rustc_smir/src/stable_mir/compiler_interface.rs

@@ -9,7 +9,6 @@ use rustc_hir::def::DefKind;
 use rustc_smir::context::SmirCtxt;
 use rustc_smir::{Bridge, SmirContainer};
 use stable_mir::abi::{FnAbi, Layout, LayoutShape, ReprOptions};
-use stable_mir::convert::{RustcInternal, Stable};
 use stable_mir::crate_def::Attribute;
 use stable_mir::mir::alloc::{AllocId, GlobalAlloc};
 use stable_mir::mir::mono::{Instance, InstanceDef, StaticDef};
@@ -21,9 +20,10 @@ use stable_mir::ty::{
     Generics, ImplDef, ImplTrait, IntrinsicDef, LineInfo, MirConst, PolyFnSig, RigidTy, Span,
     TraitDecl, TraitDef, Ty, TyConst, TyConstId, TyKind, UintTy, VariantDef, VariantIdx,
 };
+use stable_mir::unstable::{RustcInternal, Stable, new_item_kind};
 use stable_mir::{
     AssocItems, Crate, CrateDef, CrateItem, CrateItems, CrateNum, DefId, Error, Filename,
-    ImplTraitDecls, ItemKind, Symbol, TraitDecls, alloc, mir, new_item_kind,
+    ImplTraitDecls, ItemKind, Symbol, TraitDecls, alloc, mir,
 };
 use tracing::debug;