The logic in type_unification_real for handling template parms that
depend on earlier template parms is a bit complicated.  It already
recognizes when the type of the parm depends on something not
available yet, and it dealt with the case where substituting partial
args left some template parm uses behind, but it didn't handle the
case where substituting partial args just failed.

Tested x86_64-pc-linux-gnu, applying to trunk.