PR18401: Fix assert by implementing the current proposed direction of core
issue 1430. Don't allow a pack expansion to be used as an argument to an alias
template unless the corresponding parameter is a parameter pack.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198833 91177308-0d34-0410-b5e6-96231b3b80d8