This is primarily motivated by the desire to move from Python2 to
Python3. `PYTHON_EXECUTABLE` is ambiguous. This explicitly identifies
the python interpreter in use. Since the LLVM build seems to be able to
completed successfully with python3, use that across the build. The old
path aliases `PYTHON_EXECUTABLE` to be treated as Python3.
This follows in line with a previous patch of renaming LLVM's.
Working on these files is difficult in certain operating systems
and/or environments that don't like handling python code with a
non .py file extension.
llvm-svn: 313892
Saleem Abdulrasool