[CMake] Fail when Python interpreter doesn't match Python libraries version

Because of how CMake finds the Python libraries and interpreter, it's
possible to end up with a discrepancy between the two. For example,
you'd end up using a Python 3 interpreter to run the test suite while
LLDB was built and linked against Python 2.

This patch adds a fatal error to CMake so we find out at configuration
time, instead of finding out at test time.

Differential revision: https://reviews.llvm.org/D64812

llvm-svn: 366243