Enable building LTO on WIN32.

Enable building the LTO library (.lib and.dll) and llvm-lto.exe on Windows with
MSVC and Mingw as well as re-enabling the associated test.

Patch by Greg Bedwell!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191823 91177308-0d34-0410-b5e6-96231b3b80d8