swift234

[driver] Added support for specifying output levels.

Added an OutputLevel enum, which is used by Compilation to determine what kind
of output it should emit when performing Jobs.

In Driver::buildCompilation(), set the default OutputLevel for the Compilation
to Normal, but set it to Verbose if -v was passed. This means that, by default,
the driver will only print subtask output; this matches Clang's behavior.

Swift SVN r13255