Serialized diagnostic severity levels should be stable.

Serialized diagnostics were accidentally using the AST diagnostic level values
rather than a dedicated stable enum, so the addition of "remark" broke the
reading of existing serialized diagnostics files. I've added a .dia file
generated from Xcode 5's Clang to make sure we don't break this in the future.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202733 91177308-0d34-0410-b5e6-96231b3b80d8