Change two class forward declarations to struct forward declarations, silencing a Clang warning

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@99405 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/Mangle.h b/lib/CodeGen/Mangle.h
index 759cfc3..34b2ab4 100644
--- a/lib/CodeGen/Mangle.h
+++ b/lib/CodeGen/Mangle.h
@@ -34,8 +34,8 @@
   class VarDecl;
 
 namespace CodeGen {
-  class ThisAdjustment;
-  class ThunkInfo;
+  struct ThisAdjustment;
+  struct ThunkInfo;
   class CovariantThunkAdjustment;
   class ThunkAdjustment;