[cl] Don't print subcommand help when no subcommands present.

Previously we would print

  USAGE: <exe> [subcommand] [options]

Even if no subcommands were present.  This changes the output
format to only print "[subcommand]" if there is at least one
subcommand.

Fixes llvm.org/pr30598

Patch by Serge Guelton

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@283892 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/CommandLine.cpp

@@ -1756,10 +1756,12 @@ class HelpPrinter {
     if (!GlobalParser->ProgramOverview.empty())
       outs() << "OVERVIEW: " << GlobalParser->ProgramOverview << "\n";
 
-    if (Sub == &*TopLevelSubCommand)
-      outs() << "USAGE: " << GlobalParser->ProgramName
-             << " [subcommand] [options]";
-    else {
+    if (Sub == &*TopLevelSubCommand) {
+      outs() << "USAGE: " << GlobalParser->ProgramName;
+      if (Subs.size() > 2)
+        outs() << " [subcommand]";
+      outs() << " [options]";
+    } else {
       if (!Sub->getDescription().empty()) {
         outs() << "SUBCOMMAND '" << Sub->getName()
                << "': " << Sub->getDescription() << "\n\n";