Revert "Enable building LTO on WIN32."

This reverts commit r191670.

It was causing build failures on the msvc bots:

http://bb.pgr.jp/builders/ninja-clang-i686-msc17-R/builds/5166/steps/compile/logs/stdio

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191679 91177308-0d34-0410-b5e6-96231b3b80d8