[CMake] Support for generating Xcode 8 compatible toolchains

Xcode 8 requires toolchain compatibility version 2. This allows us to select the correct compatibility version based on the installed version of Xcode.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@279152 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed