[lldb/Driver] Print snippet before exiting with unknown argument.

Print a little snippet before exiting when passed unrecognized
arguments. The goal is twofold:

 - Point users to lldb --help.
 - Make it clear that we exited the debugger.

lldb/test/Shell/Driver/TestPositionalArgs.test

@@ -28,3 +28,4 @@ RUN: not %lldb -x -b -f %t.foo bar -baz --quux 2>&1 | FileCheck %s --check-prefi
 
 UNKNOWN: error: unknown option: -baz
 UNKNOWN: error: unknown option: --quux
+UNKNOWN: Use 'lldb --help' for a complete list of options.

lldb/tools/driver/Driver.cpp

@@ -850,9 +850,10 @@ int main(int argc, char const *argv[]) {
   unsigned MAC;
   ArrayRef<const char *> arg_arr = makeArrayRef(argv + 1, argc - 1);
   opt::InputArgList input_args = T.ParseArgs(arg_arr, MAI, MAC);
+  llvm::StringRef argv0 = llvm::sys::path::filename(argv[0]);
 
   if (input_args.hasArg(OPT_help)) {
-    printHelp(T, llvm::sys::path::filename(argv[0]));
+    printHelp(T, argv0);
     return 0;
   }
 
@@ -861,6 +862,8 @@ int main(int argc, char const *argv[]) {
     for (auto *arg : input_args.filtered(OPT_UNKNOWN)) {
       WithColor::error() << "unknown option: " << arg->getSpelling() << '\n';
     }
+    llvm::errs() << "Use '" << argv0
+                 << " --help' for a complete list of options.\n";
     return 1;
   }