ADT: Fix build after r281182

Fix this bot:
  http://lab.llvm.org:8011/builders/clang-x86_64-linux-abi-test/builds/20680

which complained that a typedef of 'iterator_adaptor_base' changed
meaning in AllocatorList::IteratorImpl.  Use 'base_type' there instead.

llvm-svn: 281183