I started looking at allowing non-type constrained-type-specifiers in 
auto deduction and then realized that we didn't handle them in function 
parameters either.  Fixing that brought home to me the oddity of having 
a type-specifier stand in for a non-type.  Mind weighing in on that on 
the core reflector?

I also wonder why we have two different ways of expressing a 
constrained-type-specifier in the implementation: a constrained 
template-parameter (TYPE_DECL, is_constrained_parameter) and a 
constrained auto (TEMPLATE_TYPE_PARM).  Why not represent them the same way?

This patch doesn't mess with this duality, but extends 
equivalent_placeholder_constraints to deal with both kinds, and then 
uses that for comparing constrained-type-specifiers.  And also handles 
non-type constrained-type-specifiers in abbreviated function templates.

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