Teach structured errors to display short Ty

Make it so that every structured error annotated with `#[derive(Diagnostic)]` that has a field of type `Ty<'_>`, the printing of that value into a `String` will look at the thread-local storage `TyCtxt` in order to shorten to a length appropriate with the terminal width. When this happen, the resulting error will have a note with the file where the full type name was written to.

```
error[E0618]: expected function, found `((..., ..., ..., ...), ..., ..., ...)``
 --> long.rs:7:5
  |
6 | fn foo(x: D) { //~ `x` has type `(...
  |        - `x` has type `((..., ..., ..., ...), ..., ..., ...)`
7 |     x(); //~ ERROR expected function, found `(...
  |     ^--
  |     |
  |     call expression requires function
  |
  = note: the full name for the type has been written to 'long.long-type-14182675702747116984.txt'
  = note: consider using `--verbose` to print the full type name to the console
```

compiler/rustc_middle/src/error.rs

@@ -47,10 +47,10 @@ pub struct UnsupportedUnion {
 // FIXME(autodiff): I should get used somewhere
 #[derive(Diagnostic)]
 #[diag(middle_autodiff_unsafe_inner_const_ref)]
-pub struct AutodiffUnsafeInnerConstRef {
+pub struct AutodiffUnsafeInnerConstRef<'tcx> {
     #[primary_span]
     pub span: Span,
-    pub ty: String,
+    pub ty: Ty<'tcx>,
 }
 
 #[derive(Subdiagnostic)]