Rename the unescaping functions.

`unescape_literal` becomes `unescape_unicode`, and `unescape_c_string`
becomes `unescape_mixed`. Because rfc3349 will mean that C string
literals will no longer be the only mixed utf8 literals.

clippy_dev/src/update_lints.rs

@@ -928,7 +928,7 @@ fn remove_line_splices(s: &str) -> String {
         .and_then(|s| s.strip_suffix('"'))
         .unwrap_or_else(|| panic!("expected quoted string, found `{s}`"));
     let mut res = String::with_capacity(s.len());
-    unescape::unescape_literal(s, unescape::Mode::Str, &mut |range, ch| {
+    unescape::unescape_unicode(s, unescape::Mode::Str, &mut |range, ch| {
         if ch.is_ok() {
             res.push_str(&s[range]);
         }