Fix latent clang-tidy errors

Removes two defaulted and implicitly deleted assignment
operators in the MPD client. Also fixes `Die` utility to
enable compile-time checking of format parameters.

src/mpd_client.cc

@@ -58,7 +58,6 @@ class SongImpl : public Song {
 
     // However, moves are OK, because the "old" owner no longer exists.
     SongImpl(SongImpl&&) = default;
-    SongImpl& operator=(SongImpl&&) = default;
 
     // Free the wrapped struct mpd_song;
     ~SongImpl() override;
@@ -178,7 +177,6 @@ class SongReaderImpl : public SongReader {
 
     // As with the other types, moves are OK.
     SongReaderImpl(SongReaderImpl&&) = default;
-    SongReaderImpl& operator=(SongReaderImpl&&) = default;
 
     // Default destructor should work fine, since the std::optional owns
     // a unique_ptr to an actual Song. The generated destructor will destruct

src/util.h

@@ -11,8 +11,8 @@ namespace {
 // Die logs the given message as if it was printed via `absl::StrFormat`,
 // and then terminates the program with with an error status code.
 template <typename... Args>
-void Die(Args... strs) {
-    std::cerr << absl::StrFormat(strs...) << std::endl;
+void Die(absl::FormatSpec<Args...> format, Args... vars) {
+    std::cerr << absl::StrFormat(format, vars...) << std::endl;
     std::exit(EXIT_FAILURE);
 }