[lldb/CMake] Add LLDB_PYTHON_VERSION to use Python 2 with CMake > 3.12

In addition to having the default fallback from Python 3 to Python 2, it
should also be possible to build against Python 2 explicitly. This patch
makes that possible by setting LLDB_PYTHON_VERSION. The variable only
has effect with CMake 3.12 or later.

Differential revision: https://reviews.llvm.org/D81501