Ensure comparison of constraints creates a code synth context

This is a regression from 6db007a0 that was reported in:
https://github.com/llvm/llvm-project/issues/62697

The assertion was because we require a code synthesis context for the
instantiation of templates, and this reproducer causes a comparison that
doesn't have a parent-template causing one to exists.

This patch fixes it by creating a ConstraintNormalization context.