[llvm] [Support] Show '[subcommand]' in the help for less than 3 subcommands (PR #74557)

[Igor Kudrin via llvm-commits]

igorkudrin wrote:

I can't find an easy way to test this change. `cl::PrintHelpMessage()` uses `llvm::outs()` unconditionally; changing it to take an output stream as an argument would cause a cascade of changes in all the methods it calls directly and indirectly, including, for example, `Option::printOptionInfo()` with its overrides, and many others. The patch doesn't seem worth such a massive change.

PS The buildbot failures do not seem relevant, as the patch could not cause crashes.

https://github.com/llvm/llvm-project/pull/74557