Stop setting LLVM_ON_WIN32 in config.h and llvm-config.h.

See thread "Replacing LLVM_ON_WIN32 with just _WIN32" on llvm-dev and cfe-dev.

I replaced all uses of LLVM_ON_WIN32 with _WIN32 in r331127 (llvm),
r331069 (clang), r329697 (lldb), r329696 (lld), r329696 (clang-tools-extra).

If your out-of-tree program used LLVM_ON_WIN32, just use _WIN32 instead, which
is set at exactly the same time to exactly the same value.

https://reviews.llvm.org/D46264

llvm-svn: 331224