Recommit r287403 (reverted in r287804): [lit] When setting SDKROOT on Darwin,...

Recommit r287403 (reverted in r287804): [lit] When setting SDKROOT on Darwin, use '--sdk macosx' to find the right SDK path.

This shouls now be safe and not break any more bots.  It's strictly better to use '--sdk macosx', otherwise xcrun can return weird things for example when you have Command Line Tools or the SDK installed into '/'.

llvm-svn: 288385