Use the correct Python lib for each build configuration generated by the...

Use the correct Python lib for each build configuration generated by the Visual Studio CMake generator

Summary:
Previously `CMAKE_BUILD_TYPE` was used to determine whether to link in `python27.lib` or `python27_d.lib`, unfortunately this only works reliably when using a CMake generator that generates a single build configuration (e.g. Ninja). The Visual Studio CMake generator generates four build configurations at once (`Debug`, `Release`, `RelWithDebInfo`, `MinSizeRel`), so if `CMAKE_BUILD_TYPE` is set to `Debug` all four build configurations end up linking in `python27_d.lib`, this is clearly undesirable.

To ensure that the correct Python lib is used for each build configuration the value of `PYTHON_LIBRARY` is now determined using generator expressions that evaluate to either the debug or release Python lib. The values of `PYTHON_EXECUTABLE` and `PYTHON_DLL` are now likewise determined using generator expressions.

Note that these changes only apply to the Windows build.

Patch by Vadim Macagon. Thanks!

Reviewers: zturner, brucem

Subscribers: zturner, lldb-commits

Differential Revision: http://reviews.llvm.org/D13234

llvm-svn: 248991