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
diff --git a/lib/Parse/ParseTentative.cpp b/lib/Parse/ParseTentative.cpp
index 8a5e726..41d793f 100644
--- a/lib/Parse/ParseTentative.cpp
+++ b/lib/Parse/ParseTentative.cpp
@@ -735,6 +735,7 @@
   case tok::kw_alignof:
   case tok::kw_noexcept:
   case tok::kw_nullptr:
+  case tok::kw__Alignof:
   case tok::kw___null:
   case tok::kw___alignof:
   case tok::kw___builtin_choose_expr: