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

rG01f9abbb50b1 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.

(cherry picked from commit 0b6abe4)

Author: Bigcheese

llvm/include/llvm/module.modulemap

@@ -380,6 +380,9 @@ module LLVM_Utils {
 
     umbrella "Support"
     module * { export * }
+    
+    // Exclude this; it should only be used on Windows.
+    exclude header "Support/Windows/WindowsSupport.h"
 
     // Exclude these; they are fundamentally non-modular.
     exclude header "Support/PluginLoader.h"