Bring back magic constants in the digraph diagnostic

This backs out changes in commit r198605 and part of r198604, replacing the
original tok::kw_template with a slightly more obvious placeholder
tok::unknown.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198666 91177308-0d34-0410-b5e6-96231b3b80d8