Revert "[lit] When setting SDKROOT on Darwin, use '--sdk macosx' to find the right SDK path."
Revert "[lit] When setting SDKROOT on Darwin, use '--sdk macosx' to find the right SDK path."

This reverts commit r287403. It breaks an internal asan bot. According
to Kuba, a fix is up for review here: https://reviews.llvm.org/D26929

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@287804 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm