When passing no arguments to compile suite, show an error instead of crashing

The command "lnt runtest compile" crashes, where as "lnt runtest nt" prints help. Fix this so the compile suite also displays help.

git-svn-id: https://llvm.org/svn/llvm-project/lnt/trunk@205927 91177308-0d34-0410-b5e6-96231b3b80d8