Make LLVMContext and LLVMContextImpl classes instead of structs.

llvm-svn: 78690
diff --git a/llvm/lib/VMCore/ConstantFold.h b/llvm/lib/VMCore/ConstantFold.h
index c70a04b..afa9978 100644
--- a/llvm/lib/VMCore/ConstantFold.h
+++ b/llvm/lib/VMCore/ConstantFold.h
@@ -23,7 +23,7 @@
   class Value;
   class Constant;
   class Type;
-  struct LLVMContext;
+  class LLVMContext;
 
   // Constant fold various types of instruction...
   Constant *ConstantFoldCastInstruction(