Fix windows-x86-debug compilation with python enabled using multi-target generator

[Patch by Leonid Mashinskiy]

Visual Studio CMake generator is multi-target and does not define
CMAKE_BUILD_TYPE, so Debug build on VS was failing due selection of release
python library. This patch reverts back some of latest changes and fixes
building by raw VS using CMake expression generators.

Differential Revision: https://reviews.llvm.org/D66994

llvm-svn: 371090