Fix the DeclContextLookupResult::iterator non-copyable.

[hokein]

Summary

The value_type is a const pointer, which makes the iteator non-copyable.
Before the patch, the normal usage like below was illegal:

auto It = lookupresult.begin();
...
It = lookupresult.end(); // the copy is not allowed.

[sammccall]

clang/include/clang/AST/DeclBase.h

1249

I wonder if this was meant to be const NamedDecl * - note that begin()/end() are const.

However I'm not sure (clang AST is always funny about const) and that would be a breaking change.

clang/include/clang/AST/DeclBase.h

DeclContextLookupResult(ArrayRef<NamedDecl *> Result)
       : Result(Result) {}
   DeclContextLookupResult(NamedDecl *Single)
       : Result(SingleElementDummyList), Single(Single) {}
 
   class iterator;
 
   using IteratorBase =
       llvm::iterator_adaptor_base<iterator, ResultTy::iterator,
-                                  std::random_access_iterator_tag,
-                                  NamedDecl *const>;
+                                  std::random_access_iterator_tag, NamedDecl *>;
 
   class iterator : public IteratorBase {
     value_type SingleElement;
 
   public:
     explicit iterator(pointer Pos, value_type Single = nullptr)
         : IteratorBase(Pos), SingleElement(Single) {}