Reserve prefixed identifiers and string literals (RFC 3101)

This commit denies any identifiers immediately followed by
one of three tokens `"`, `'` or `#`, which is stricter than
the requirements of RFC 3101 but may be necessary according
to the discussion at [Zulip].

[Zulip]: https://rust-lang.zulipchat.com/#narrow/stream/268952-edition-2021/topic/reserved.20prefixes/near/238470099

src/librustdoc/html/highlight.rs

@@ -413,7 +413,7 @@ impl<'a> Classifier<'a> {
                 },
                 c => c,
             },
-            TokenKind::RawIdent => Class::Ident,
+            TokenKind::RawIdent | TokenKind::BadPrefix => Class::Ident,
             TokenKind::Lifetime { .. } => Class::Lifetime,
         };
         // Anything that didn't return above is the simple case where we the