Add -debug-diagnostic-names frontend flag

This flag adds diagnostic names to the end of their messages, e.g. 'error: cannot convert value of type '[Any]' to specified type '[Int]' [cannot_convert_initializer_value]'. It's intended to be used for debugging purposes when working on the compiler.

lib/Frontend/CompilerInvocation.cpp

@@ -669,6 +669,7 @@ static bool ParseDiagnosticArgs(DiagnosticOptions &Opts, ArgList &Args,
   Opts.FixitCodeForAllDiagnostics |= Args.hasArg(OPT_fixit_all);
   Opts.SuppressWarnings |= Args.hasArg(OPT_suppress_warnings);
   Opts.WarningsAsErrors |= Args.hasArg(OPT_warnings_as_errors);
+  Opts.PrintDiagnosticNames |= Args.hasArg(OPT_debug_diagnostic_names);
 
   assert(!(Opts.WarningsAsErrors && Opts.SuppressWarnings) &&
          "conflicting arguments; should have been caught by driver");