Add ErrorGuaranteed to ast::LitKind::Err, token::LitKind::Err.

This mostly works well, and eliminates a couple of delayed bugs.

One annoying thing is that we should really also add an
`ErrorGuaranteed` to `proc_macro::bridge::LitKind::Err`. But that's
difficult because `proc_macro` doesn't have access to `ErrorGuaranteed`,
so we have to fake it.

compiler/rustc_ast/src/ast.rs

@@ -1846,7 +1846,7 @@ pub enum LitKind {
     /// A boolean literal (`true`, `false`).
     Bool(bool),
     /// Placeholder for a literal that wasn't well-formed in some way.
-    Err,
+    Err(ErrorGuaranteed),
 }
 
 impl LitKind {
@@ -1893,7 +1893,7 @@ impl LitKind {
             | LitKind::Int(_, LitIntType::Unsuffixed)
             | LitKind::Float(_, LitFloatType::Unsuffixed)
             | LitKind::Bool(..)
-            | LitKind::Err => false,
+            | LitKind::Err(_) => false,
         }
     }
 }