Fix the Windows build after r273599

That patch made all LLVM projects build with -DUNICODE. However, this doesn't
work for the OpenMP runtime.

But just overriding the flag with -UUNICODE breaks compiling ittnotify_static.c,
which for some reason needs to be compiled with -DUNICIODE. Note that compiling
ittnotify.h with -DUNICODE does not work though.

This seems like a mess. This commit fixes it for now, but it would be great
if someone who works on the OpenMP runtime could fix it properly.

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@273898 91177308-0d34-0410-b5e6-96231b3b80d8