PR21536: Fix a corner case where we'd get confused by a pack expanding into the
penultimate parameter of a template parameter list, where the last parameter is
itself a pack, and build a bogus empty final pack argument.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221748 91177308-0d34-0410-b5e6-96231b3b80d8