swift234

[Driver] Allow falling back to / for -i mode if xcrun says to.

After a long discussion with Daniel, the behavior of "xcrun --sdk macosx"
will always pick the best  SDK to use with the Swift found by "xcrun swift".
The only case in which we have a problem is if someone has explicitly
specified the path to a Swift binary, or if someone is on 10.9 but only has
the command line tools installed (in which case they lose anyway).

In order to keep this from polluting the tests, I've changed %swift_driver
to set SDKROOT to an empty path by default. Use -sdk if you need to provide
an SDK in a test, not the SDKROOT environment variable. (This is what
everyone has been doing so far.)

This is still limited to -i and REPL modes; for compilation an explicit SDK
is still required.

<rdar://problem/14395800>

Swift SVN r18770