Implement C++11 [dcl.align]p1 and C11 6.7.5/2 rules for alignas and _Alignas.

llvm-svn: 173779
diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 438c6f8..20d8f71 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -3890,6 +3890,9 @@
   case tok::kw_explicit:
   case tok::kw__Noreturn:
 
+    // alignment-specifier
+  case tok::kw__Alignas:
+
     // friend keyword.
   case tok::kw_friend: