[Clang] use non-instantiated function declaration for constraints partial ordering

Per wordings in
- https://eel.is/c++draft/over.match#best.general-2.6
- https://eel.is/c++draft/temp.constr.order
- https://eel.is/c++draft/temp.constr#atomic-1

constraints partial ordering should use the unsubstituted template
parameters of the constrained entity, not the instantiated entity.

Fix #56154

Reviewed By: erichkeane, royjacobson, mizvekov

Differential Revision: https://reviews.llvm.org/D136545