Without this patch, failing to provide a subcommand to lit's internal
`env` results in either a python `IndexError` or an attempt to execute
the final `env` argument, such as `FOO=1`, as a command. This patch
diagnoses those cases with a more helpful message.
Reviewed By: stella.stamenova
Differential Revision: https://reviews.llvm.org/D66482
llvm-svn: 369620