[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

llvm-svn: 283892