[clang][Sema] Use original template pattern when declaring implicit deduction...

[clang][Sema] Use original template pattern when declaring implicit deduction guides for nested template classes (#68379)

When a nested template is instantiated, the template pattern of the
inner class is not copied into the outer class
ClassTemplateSpecializationDecl. The specialization contains a
ClassTemplateDecl with an empty record that points to the original
template pattern instead.

As a result, when looking up the constructors of the inner class, no
results are returned. This patch finds the original template pattern and
uses that for the lookup instead.

Based on CWG2471 we must also substitute the known outer template
arguments when creating deduction guides for the inner class.

Fixes #46200
Fixes #57812