Look for Python 2 before Python 3 in CMakeLists.txt

All of LLVM's Python scripts only support Python 2 for widely understood
reasons.

Patch by Yonggang Luo.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184732 91177308-0d34-0410-b5e6-96231b3b80d8