[Concepts] Fix incorrect check when instantiating abbreviated template type-constraints
We would incorrectly check whether the type-constraint had already been initialized, causing us
to ignore the invented template type constraints entirely.
Also, TemplateParameterList would store incorrect information about invented type parameters
when it observed them before their type-constraint was initialized, so we recreate it after
initializing the function type of an abbreviated template.
(cherry picked from commit 38fd69995fc5a6f16e0aa132a46e5ccdbc2eebb3)