[clang] AST: SubstTemplateTypeParmType support for non-canonical underlying type

This change allows us to represent in the AST some specific
circumstances where we substitute a template parameter type
which is part of the underlying type of a previous substitution.

This presently happens in some circumstances dealing with
substitution of defaulted parameters of template template
parameters, and in some other cases during concepts substitution.

The main motivation for this change is for the future use in the
implementation of template specialization resugaring, as this will
allow us to represent a substitution with sugared types.

Signed-off-by: Matheus Izvekov <mizvekov@gmail.com>

Differential Revision: https://reviews.llvm.org/D132816