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/ParseDecl.cpp b/lib/Parse/ParseDecl.cpp
index 409f8e0..c6db497 100644
--- a/lib/Parse/ParseDecl.cpp
+++ b/lib/Parse/ParseDecl.cpp
@@ -2478,7 +2478,7 @@
     // alignment-specifier
     case tok::kw__Alignas:
       if (!getLangOpts().C11)
-        Diag(Tok, diag::ext_c11_alignas);
+        Diag(Tok, diag::ext_c11_alignment) << Tok.getName();
       ParseAlignmentSpecifier(DS.getAttributes());
       continue;