swift234

[Driver] Factor interpret-script mode out into a separate Action.

This makes it easier to make interpreter modes behave differently from
compilation modes. Obviously that's a trade-off, since the two modes also
share plenty, but given how few of the existing CompileJobAction checks had
to be modified for the new InterpretJobAction, I think this is the right
way to go.

Groundwork for setting [DY]LD_LIBRARY_PATH ahead of time when invoking the
interpreter, which is rdar://problem/23588774.