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

CMakeLists.txt

@@ -250,7 +250,8 @@ set(TARGET_TRIPLE "${LLVM_DEFAULT_TARGET_TRIPLE}")
 
 include(HandleLLVMOptions)
 
-# Verify that we can find a Python interpreter,
+# Verify that we can find a Python 2 interpreter.  Python 3 is unsupported.
+set(Python_ADDITIONAL_VERSIONS 2.7 2.6 2.5 2.4)
 include(FindPythonInterp)
 if( NOT PYTHONINTERP_FOUND )
   message(FATAL_ERROR