As correctly pointed out by Martin on the mailing list, Python should
only be auto-enabled if SWIG is found as well. This moves the logic of
finding SWIG into FindPythonInterpAndLibs to make that possible.
To make diagnosing easier I've included a status message to convey why
Python support is disabled.
Python was the last remaining "optional" dependency for LLDB. This moves
the code to find Python into FindPythonInterpAndLibs using the same
principles as FindCursesAndPanel.
Differential revision: https://reviews.llvm.org/D72107