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/src/ast.rs

@@ -1422,7 +1422,7 @@ pub enum ExprKind {
     /// of `if` / `while` expressions. (e.g., `if let 0 = x { .. }`).
     ///
     /// `Span` represents the whole `let pat = expr` statement.
-    Let(P<Pat>, P<Expr>, Span, Option<ErrorGuaranteed>),
+    Let(P<Pat>, P<Expr>, Span, Recovered),
     /// An `if` block, with an optional `else` block.
     ///
     /// `if expr { block } else { expr }`
@@ -2881,17 +2881,20 @@ pub struct FieldDef {
     pub is_placeholder: bool,
 }
 
+/// Was parsing recovery performed?
+#[derive(Copy, Clone, Debug, Encodable, Decodable, HashStable_Generic)]
+pub enum Recovered {
+    No,
+    Yes(ErrorGuaranteed),
+}
+
 /// Fields and constructor ids of enum variants and structs.
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub enum VariantData {
     /// Struct variant.
     ///
     /// E.g., `Bar { .. }` as in `enum Foo { Bar { .. } }`.
-    Struct {
-        fields: ThinVec<FieldDef>,
-        // FIXME: investigate making this a `Option<ErrorGuaranteed>`
-        recovered: bool,
-    },
+    Struct { fields: ThinVec<FieldDef>, recovered: Recovered },
     /// Tuple variant.
     ///
     /// E.g., `Bar(..)` as in `enum Foo { Bar(..) }`.