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/AST/StmtDumper.cpp b/lib/AST/StmtDumper.cpp
index ce330e0..d5e12c8 100644
--- a/lib/AST/StmtDumper.cpp
+++ b/lib/AST/StmtDumper.cpp
@@ -463,7 +463,7 @@
     OS << " sizeof ";
     break;
   case UETT_AlignOf:
-    OS << " __alignof ";
+    OS << " alignof ";
     break;
   case UETT_VecStep:
     OS << " vec_step ";