s/LLVM_ON_WIN32/_WIN32/, clang

s/LLVM_ON_WIN32/_WIN32/, clang

LLVM_ON_WIN32 is set exactly with MSVC and MinGW (but not Cygwin) in
HandleLLVMOptions.cmake, which is where _WIN32 defined too.  Just use the
default macro instead of a reinvented one.

See thread "Replacing LLVM_ON_WIN32 with just _WIN32" on llvm-dev and cfe-dev.
No intended behavior change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@331069 91177308-0d34-0410-b5e6-96231b3b80d8