Reorder FindPythonInterp so that config-ix can use PYTHON_EXECUTABLE

Summary:
Code in config-ix tries to call `PYTHON_EXECUTABLE` to search for some
python modules but that variable isn't set until the moved chunk of
code that finds Python is called.

Reorder it so CMake can use PYTHON_EXECUTABLE

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D52763

llvm-svn: 346367