Retain constraints in the canonical form of an auto type.

This is necessary in order for type equality checking, for example
across redeclarations, to require constraints to match. This is also a
prerequisite for including the constraints in manglings.

In passing, fix a bug where TemplateArgument::Profile would produce the
same profile for two structurally different template names, which caused
this change to re-expose the crash previously addressed by D133072,
which it turns out had not quite addressed all problematic cases.