ADT: Fix build after r281182
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.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@281183 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 8650ea18c2ccf6f4a4dc94bdf5c30d678cb064ef)

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm