Revert "[CMake] Don't pass in MSVC warning flags as definitions"

reverting while I investigate why it broke the sanitizer-windows build.
This reverts commit r231924.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231925 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed