I just realized that the specialized metadata node patch I'm about to
commit won't compile on old compilers. Bump `hash_combine()`'s support
for non-variadic templates to 18 (I tested this by reversing the logic
in the #ifdef).
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228629 91177308-0d34-0410-b5e6-96231b3b80d8