Rollup merge of #126567 - compiler-errors:instance-kind, r=oli-obk,lcnr

Rename `InstanceDef` -> `InstanceKind`

Renames `InstanceDef` to `InstanceKind`. The `Def` here is confusing, and makes it hard to distinguish `Instance` and `InstanceDef`. `InstanceKind` makes this more obvious, since it's really just describing what *kind* of instance we have.

Not sure if this is large enough to warrant a types team MCP -- it's only 53 files. I don't personally think it does, but happy to write one if anyone disagrees. cc ``@rust-lang/types``

r? types

src/tools/miri/src/intrinsics/mod.rs

@@ -57,7 +57,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                     );
                 }
                 Ok(Some(ty::Instance {
-                    def: ty::InstanceDef::Item(instance.def_id()),
+                    def: ty::InstanceKind::Item(instance.def_id()),
                     args: instance.args,
                 }))
             }