Improve our handling of non-type template parameters in partial

specializations. We weren't dealing with any of the cases where the
type of the non-type template argument differs from the type of the
corresponding template parameter in the primary template. We would
think that the template parameter in the partial specialization was
not deducible (and warn about it, incorrectly), then fail to convert a
deduced parameter to the type of the template parameter in the partial
specialization (which may involve truncation, among other
things). Fixes PR8905.

llvm-svn: 122851