Try to work around an MSVC 2013 bug around defaulted default ctors

An UnresolvedSetIterator() is supposed to be zeroed out, but MSVC 2013
does not do that.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@278842 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/AST/UnresolvedSet.h

@@ -38,7 +38,9 @@ class UnresolvedSetIterator : public llvm::iterator_adaptor_base<
       : iterator_adaptor_base(const_cast<DeclAccessPair *>(Iter)) {}
 
 public:
-  UnresolvedSetIterator() = default;
+  // Work around a bug in MSVC 2013 where explicitly default constructed
+  // temporaries with defaulted ctors are not zero initialized.
+  UnresolvedSetIterator() : iterator_adaptor_base(nullptr) {}
 
   NamedDecl *getDecl() const { return I->getDecl(); }
   void setDecl(NamedDecl *ND) const { return I->setDecl(ND); }