Re-add the Metadata.h include to LLVMContextImpl.h so that MDNode is complete
where FoldingSet<MDNode> is instantiated.  Clang and MSVC complain; gcc
doesn't.

llvm-svn: 99147
diff --git a/llvm/lib/VMCore/LLVMContextImpl.h b/llvm/lib/VMCore/LLVMContextImpl.h
index 034f3b9..8666f45 100644
--- a/llvm/lib/VMCore/LLVMContextImpl.h
+++ b/llvm/lib/VMCore/LLVMContextImpl.h
@@ -21,6 +21,7 @@
 #include "llvm/LLVMContext.h"
 #include "llvm/Constants.h"
 #include "llvm/DerivedTypes.h"
+#include "llvm/Metadata.h"
 #include "llvm/Assembly/Writer.h"
 #include "llvm/Support/ValueHandle.h"
 #include "llvm/ADT/APFloat.h"
@@ -35,8 +36,6 @@
 
 class ConstantInt;
 class ConstantFP;
-class MDString;
-class MDNode;
 class LLVMContext;
 class Type;
 class Value;