swift234

Switch over tests to run with the new driver.

Everything currently using "%swift" now invokes "swift_driver -frontend",
which is the new driver's equivalent of "clang -cc1".

Add a SWIFT_NEW_DRIVER mode to the CMake build.

Building with this set to ON will use the new driver for everything.
In this mode, tests currently using "%swift" will now invoke
"swift_driver -frontend", which is the new driver's equivalent of
"clang -cc1".

The only real change here is that the new driver uses "--" to pass arguments
to interpreted files, while the old one used "--args". Tests should use "--".

This will become the default very soon.

Swift SVN r13266