Refactor variance diagnostics to work with more types

Instead of special-casing mutable pointers/references, we
now support general generic types (currently, we handle
`ty::Ref`, `ty::RawPtr`, and `ty::Adt`)

When a `ty::Adt` is involved, we show an additional note
explaining which of the type's generic parameters is
invariant (e.g. the `T` in `Cell<T>`). Currently, we don't
explain *why* a particular generic parameter ends up becoming
invariant. In the general case, this could require printing
a long 'backtrace' of types, so doing this would be
more suitable for a follow-up PR.

We still only handle the case where our variance switches
to `ty::Invariant`.

compiler/rustc_middle/src/ty/mod.rs

@@ -77,7 +77,7 @@ pub use self::sty::{
     GeneratorSubsts, GeneratorSubstsParts, InlineConstSubsts, InlineConstSubstsParts, ParamConst,
     ParamTy, PolyExistentialProjection, PolyExistentialTraitRef, PolyFnSig, PolyGenSig,
     PolyTraitRef, ProjectionTy, Region, RegionKind, RegionVid, TraitRef, TyKind, TypeAndMut,
-    UpvarSubsts, VarianceDiagInfo, VarianceDiagMutKind,
+    UpvarSubsts, VarianceDiagInfo,
 };
 pub use self::trait_def::TraitDef;