Merging r343369:

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r343369 | vitalybuka | 2018-09-28 19:17:12 -0700 (Fri, 28 Sep 2018) | 1 line

[cxx2a] Fix warning triggered by r343285
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llvm-svn: 348451

Author: tstellar

clang/include/clang/AST/DeclBase.h

@@ -1216,7 +1216,6 @@ class DeclContextLookupResult {
     value_type SingleElement;
 
   public:
-    iterator() = default;
     explicit iterator(pointer Pos, value_type Single = nullptr)
         : IteratorBase(Pos), SingleElement(Single) {}