We previously would not correctly for the initial parameter mapping for variadic template parameters in Concepts.
Testing this lead to the discovery that with the normalization process we would need to substitute into already-substituted-into
template arguments, which means we need to add NonTypeTemplateParmExpr support to TemplateInstantiator.
We do that by substituting into the replacement and the type separately, and then re-checking the expression against the NTTP
with the new type, in order to form any new required implicit casts (for cases where the type of the NTTP was dependent).
Saar Raz