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.

llvm-svn: 159494
diff --git a/clang/lib/Sema/SemaLookup.cpp b/clang/lib/Sema/SemaLookup.cpp
index 827cfbc..94efb2f 100644
--- a/clang/lib/Sema/SemaLookup.cpp
+++ b/clang/lib/Sema/SemaLookup.cpp
@@ -3604,6 +3604,12 @@
         Consumer.addKeywordResult("nullptr");
       }
     }
+
+    if (SemaRef.getLangOpts().C11) {
+      // FIXME: We should not suggest _Alignof if the alignof macro
+      // is present.
+      Consumer.addKeywordResult("_Alignof");
+    }
   }
 
   if (CCC.WantRemainingKeywords) {