PR13136:

 * When substituting a reference to a non-type template parameter pack where the
   corresponding argument is a pack expansion, transform into an expression
   which contains an unexpanded parameter pack rather than into an expression
   which contains a pack expansion. This causes the SubstNonTypeTemplateParmExpr
   to be inside the PackExpansionExpr, rather than outside, so the expression
   still looks like a pack expansion and can be deduced.

 * Teach MarkUsedTemplateParameters that we can deduce a reference to a template
   parameter if it's wrapped in a SubstNonTypeTemplateParmExpr (such nodes are
   added during alias template substitution).

llvm-svn: 159922