Fix PR#20471. Add a cast in __align_it to ensure that the bit twiddling is done at the correct size. A better solution, IMHO, would be to declare vector<bool>::__bits_per_word as 'size_type', rather than 'unsigned', but that's a possible ABI change.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@214088 91177308-0d34-0410-b5e6-96231b3b80d8