[CMake] Updating Xcode Toolchain creation to support Xcode 7

Recent changes to Xcode have changed the structure of Xcode toolchains. This patch makes the xcode-toolchain goop construct a new-format Xcode toolchain that is compatible with Xcode 7.

The new format has a compatibility version key, so when a new format comes out we can support multiple formats in parallel.

llvm-svn: 276718