[ADT] Change iterator_adaptor_base's default template arguments to forward more underlying typedefs

[timshen]

Summary

The corresponding LLVM change: D23217.

LazyVector::iterator breaks, because int isn't an iterator type.
Since iterator_adaptor_base shouldn't be blamed to break at the call to
iterator_traits<int>::xxx, I'd rather "fix" LazyVector::iterator.

The perfect solution is to model "relative pointer", but it's beyond the goal of this patch.

include/clang/AST/ExternalASTSource.h

public:
   /// values (corresponding to loaded entities), so that position -M
   /// corresponds to element 0 in the loaded entities vector, position -M+1
   /// corresponds to element 1 in the loaded entities vector, etc. This
   /// gives us a reasonably efficient, source-order walk.
   ///
   /// We define this as a wrapping iterator around an int. The
   /// iterator_adaptor_base class forwards the iterator methods to basic integer
   /// arithmetic.
-  class iterator : public llvm::iterator_adaptor_base<
-                       iterator, int, std::random_access_iterator_tag, T, int> {
+  class iterator
+      : public llvm::iterator_adaptor_base<
+            iterator, int, std::random_access_iterator_tag, T, int, T *, T &> {
     LazyVector *Self;
 
     iterator(LazyVector *Self, int Position)
         : iterator::iterator_adaptor_base(Position), Self(Self) {}
 
     bool isLoaded() const { return this->I < 0; }
     friend class LazyVector;