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/expr.rs

@@ -158,13 +158,13 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     let ohs = self.lower_expr(ohs);
                     hir::ExprKind::AddrOf(*k, *m, ohs)
                 }
-                ExprKind::Let(pat, scrutinee, span, is_recovered) => {
+                ExprKind::Let(pat, scrutinee, span, recovered) => {
                     hir::ExprKind::Let(self.arena.alloc(hir::LetExpr {
                         span: self.lower_span(*span),
                         pat: self.lower_pat(pat),
                         ty: None,
                         init: self.lower_expr(scrutinee),
-                        is_recovered: *is_recovered,
+                        recovered: *recovered,
                     }))
                 }
                 ExprKind::If(cond, then, else_opt) => {