[llvm][Support][modulemap] Exclude WindowsSupport.h from the LLVM_Util module

rG01f9abbb moved WindowsSupport.h to include/llvm/Support/Windows/

This is a problem because the modulemap include all of the Support and
ADT directories, thus any use of any header in Support or ADT would
cause the compiler to try to build WindowsSupport.h, which only works
on Windows.

Fix this by explicitly excluding WindowsSupport.h from the LLVM_Util
module.