[Support][CommandLine] Display subcommands in help when there are less than 3

subcommands

This commit fixes a bug where the help output doesn't display subcommands when
a tool has less than 3 subcommands.

This change doesn't include a corresponding unittest as there is no viable way
to provide a unittest for it.

Differential Revision: https://reviews.llvm.org/D25463

llvm-svn: 283998