Unbreak VC++ build.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@31464 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Transforms/Utils/FunctionUtils.h

@@ -18,7 +18,7 @@
 
 namespace llvm {
   class BasicBlock;
-  struct DominatorSet;
+  class DominatorSet;
   class Function;
   class Loop;
 

include/llvm/Transforms/Utils/PromoteMemToReg.h

@@ -20,8 +20,8 @@
 namespace llvm {
 
 class AllocaInst;
-struct DominatorTree;
-struct DominanceFrontier;
+class DominatorTree;
+class DominanceFrontier;
 class TargetData;
 class AliasSetTracker;