Previously, the driver would unconditionally create a CompileJobAction if it was
in SingleCompile or Immediate mode. If there were no inputs, though, the driver
would end up asserting later because it could not create a Command for a
CompileJobAction with no inputs.
To match the behavior of the StandardCompile mode, avoid creating a
CompileJobAction if there are no inputs. (This change only affects -v, since it
disables the 'no input files' error which otherwise prevents this condition from
being hit.)
Additionally, added a test to check swiftc's behavior when no inputs are
provided.
Swift SVN r20900
