swift234

[driver] Fix an assertion failure when running "swiftc -v -force-single-frontend-invocation".

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