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_expand/src/placeholders.rs

@@ -174,7 +174,10 @@ pub(crate) fn placeholder(
         }]),
         AstFragmentKind::Variants => AstFragment::Variants(smallvec![ast::Variant {
             attrs: Default::default(),
-            data: ast::VariantData::Struct { fields: Default::default(), recovered: false },
+            data: ast::VariantData::Struct {
+                fields: Default::default(),
+                recovered: ast::Recovered::No
+            },
             disr_expr: None,
             id,
             ident,