Implement LWG2400 - 'shared_ptr's get_deleter() should use addressof()', and add tests. Mark LWG2400 and LWG2404 as complete

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@222161 91177308-0d34-0410-b5e6-96231b3b80d8