Sprinkle llvm_unreachable around to placate GCC's -Wreturn-type.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@159860 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/AST/Comment.cpp b/lib/AST/Comment.cpp
index 22277ad..4681d5a 100644
--- a/lib/AST/Comment.cpp
+++ b/lib/AST/Comment.cpp
@@ -69,6 +69,7 @@
 #undef COMMENT
 #undef ABSTRACT_COMMENT
   }
+  llvm_unreachable("Unknown comment kind!");
 }
 
 Comment::child_iterator Comment::child_end() const {
@@ -82,9 +83,9 @@
 #undef COMMENT
 #undef ABSTRACT_COMMENT
   }
+  llvm_unreachable("Unknown comment kind!");
 }
 
 
 } // end namespace comments
 } // end namespace clang
-