Revert r117005, WIN32 is not predefined after all.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@117026 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Basic/Targets.cpp b/lib/Basic/Targets.cpp
index 71b96f8..b58f067 100644
--- a/lib/Basic/Targets.cpp
+++ b/lib/Basic/Targets.cpp
@@ -446,7 +446,6 @@
     }
 
     Builder.defineMacro("_INTEGRAL_MAX_BITS", "64");
-    Builder.defineMacro("WIN32");
   }
 
 public: