Add ErrorGuaranteed to Recovered::Yes and use it more.

The starting point for this was identical comments on two different
fields, in `ast::VariantData::Struct` and `hir::VariantData::Struct`:
```
    // FIXME: investigate making this a `Option<ErrorGuaranteed>`
    recovered: bool
```
I tried that, and then found that I needed to add an `ErrorGuaranteed`
to `Recovered::Yes`. Then I ended up using `Recovered` instead of
`Option<ErrorGuaranteed>` for these two places and elsewhere, which
required moving `ErrorGuaranteed` from `rustc_parse` to `rustc_ast`.

This makes things more consistent, because `Recovered` is used in more
places, and there are fewer uses of `bool` and
`Option<ErrorGuaranteed>`. And safer, because it's difficult/impossible
to set `recovered` to `Recovered::Yes` without having emitted an error.

compiler/rustc_ast_lowering/src/lib.rs

@@ -1283,7 +1283,8 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         fields.iter().enumerate().map(|f| this.lower_field_def(f)),
                     );
                     let span = t.span;
-                    let variant_data = hir::VariantData::Struct { fields, recovered: false };
+                    let variant_data =
+                        hir::VariantData::Struct { fields, recovered: ast::Recovered::No };
                     // FIXME: capture the generics from the outer adt.
                     let generics = hir::Generics::empty();
                     let kind = match t.kind {