[CMake] On Windows, require manual specification of python libs.

Embedding python with MSVC is very finicky, for reasons having
to do with the operating system's CRT, the implementation of
python itself on Windows, and even bugs in CMake.

One side effect of this is that we cannot rely on FindPythonLibs
and FindPythonInterp CMake functions to locate the correct
version of Python.  We must instead manually specify the location
of PYTHON_LIBRARY and PYTHON_INCLUDE_DIR.

As a side effect, this fixes building LLDB in release mode by
specifying -DCMAKE_BUILD_TYPE=Release, which was previously
broken.

llvm-svn: 230262