unittests/Basic/FileManagerTest.cpp: Unbreak Win32, mingw and msvc.

LLVM_ON_WIN32 is defined in llvm/Config/config.h.
IMO, it might be enough with _WIN32 in most cases, LLVM_ON_xxx could be deprecated.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@133794 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed