[Diagnostics] Add -enable-experimental-descriptive-diagnostics frontend flag

This flag will feature-gate work on producing more descriptive diagnostic messages.
It will remain a hidden frontend option until these improvements are ready to ship.

lib/Frontend/CompilerInvocation.cpp

@@ -678,6 +678,8 @@ static bool ParseDiagnosticArgs(DiagnosticOptions &Opts, ArgList &Args,
   Opts.SuppressWarnings |= Args.hasArg(OPT_suppress_warnings);
   Opts.WarningsAsErrors |= Args.hasArg(OPT_warnings_as_errors);
   Opts.PrintDiagnosticNames |= Args.hasArg(OPT_debug_diagnostic_names);
+  Opts.EnableDescriptiveDiagnostics |=
+      Args.hasArg(OPT_enable_descriptive_diagnostics);
 
   assert(!(Opts.WarningsAsErrors && Opts.SuppressWarnings) &&
          "conflicting arguments; should have been caught by driver");