Add support for the C11 _Alignof keyword.

This behaves like the existing GNU __alignof and C++11 alignof keywords;
most of the patch is simply adding the third token spelling to various places.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@159494 91177308-0d34-0410-b5e6-96231b3b80d8
11 files changed