PR14995: Allow a dependent type as the second parameter of operator++ and
operator--, since it might instantiate as 'int' (or, if it's a pack, it
might instantiate as an empty pack).


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200496 91177308-0d34-0410-b5e6-96231b3b80d8