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_pretty/src/pprust/state.rs

@@ -254,7 +254,7 @@ fn literal_to_string(lit: token::Lit) -> String {
         token::CStrRaw(n) => {
             format!("cr{delim}\"{symbol}\"{delim}", delim = "#".repeat(n as usize))
         }
-        token::Integer | token::Float | token::Bool | token::Err => symbol.to_string(),
+        token::Integer | token::Float | token::Bool | token::Err(_) => symbol.to_string(),
     };
 
     if let Some(suffix) = suffix {