Create a new class: AttributeSetNode.

This is a helper class for the AttributeSetImpl class. It holds a set of
attributes that apply to a single element: function, return type, or
parameter.

These are uniqued.

llvm-svn: 173310
diff --git a/llvm/lib/IR/LLVMContextImpl.h b/llvm/lib/IR/LLVMContextImpl.h
index 30fd666..cc7ca5e 100644
--- a/llvm/lib/IR/LLVMContextImpl.h
+++ b/llvm/lib/IR/LLVMContextImpl.h
@@ -249,6 +249,7 @@
 
   FoldingSet<AttributeImpl> AttrsSet;
   FoldingSet<AttributeSetImpl> AttrsLists;
+  FoldingSet<AttributeSetNode> AttrsSetNodes;
 
   StringMap<Value*> MDStringCache;