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_hir/src/hir.rs

@@ -1308,9 +1308,9 @@ pub struct LetExpr<'hir> {
     pub pat: &'hir Pat<'hir>,
     pub ty: Option<&'hir Ty<'hir>>,
     pub init: &'hir Expr<'hir>,
-    /// `Some` when this let expressions is not in a syntanctically valid location.
+    /// `Recovered::Yes` when this let expressions is not in a syntanctically valid location.
     /// Used to prevent building MIR in such situations.
-    pub is_recovered: Option<ErrorGuaranteed>,
+    pub recovered: ast::Recovered,
 }
 
 #[derive(Debug, Clone, Copy, HashStable_Generic)]
@@ -3030,11 +3030,7 @@ pub enum VariantData<'hir> {
     /// A struct variant.
     ///
     /// E.g., `Bar { .. }` as in `enum Foo { Bar { .. } }`.
-    Struct {
-        fields: &'hir [FieldDef<'hir>],
-        // FIXME: investigate making this a `Option<ErrorGuaranteed>`
-        recovered: bool,
-    },
+    Struct { fields: &'hir [FieldDef<'hir>], recovered: ast::Recovered },
     /// A tuple variant.
     ///
     /// E.g., `Bar(..)` as in `enum Foo { Bar(..) }`.

compiler/rustc_hir/src/intravisit.rs

@@ -768,7 +768,7 @@ pub fn walk_expr<'v, V: Visitor<'v>>(visitor: &mut V, expression: &'v Expr<'v>)
         ExprKind::DropTemps(ref subexpression) => {
             try_visit!(visitor.visit_expr(subexpression));
         }
-        ExprKind::Let(LetExpr { span: _, pat, ty, init, is_recovered: _ }) => {
+        ExprKind::Let(LetExpr { span: _, pat, ty, init, recovered: _ }) => {
             // match the visit order in walk_local
             try_visit!(visitor.visit_expr(init));
             try_visit!(visitor.visit_pat(pat));