Fix PR#20471. Add a cast in __align_it to ensure that the bit twiddling is...

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.

llvm-svn: 214088