A little template argument deduction test uncovered an "oops". As part

of template instantiation, we were dropping cv-qualifiers on the
instantiated type in a few places. This change reshuffles the
type-instantiation code a little bit so that there's a single place
where we add qualifiers to the instantiated type, so that we won't end
up with this same bug in the future.

llvm-svn: 74331