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

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.

Reviewers: chandlerc, bkramer

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D23218

llvm-svn: 278156