CommandLine: Exit successfully for -version and -help

Tools that use the CommandLine library currently exit with an error
when invoked with -version or -help. This is unusual and non-standard,
so we'll fix them to exit successfully instead.

I don't expect that anyone relies on the current behaviour, so this
should be a fairly safe change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202530 91177308-0d34-0410-b5e6-96231b3b80d8