PR19372: Keep checking template arguments after we see an argument pack
expansion into a parameter pack; we know that we're still filling in that
parameter's arguments. Previously, if we hit this case for an alias template,
we'd try to substitute using non-canonical template arguments.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221832 91177308-0d34-0410-b5e6-96231b3b80d8