Add support for has_feature(cxx_alignof) and has_feature(c_alignof).

r142020 added support for has_feature(cxx_alignas). This does the same for
alignof.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223186 91177308-0d34-0410-b5e6-96231b3b80d8