Replace GetTypeInSpecific with GetTypeOfInstInSpecific. (#5232)

Reduce usage of `GetConstantInSpecific` to a single caller in constant
evaluation, with a TODO to remove that.

This gets us closer to being able to fully perform type-checking against
abstract types instead of types anchored within a particular generic.

toolchain/sem_ir/function.cpp

@@ -93,8 +93,7 @@ auto Function::GetDeclaredReturnType(const File& file,
   if (!return_slot_pattern_id.has_value()) {
     return TypeId::None;
   }
-  return GetTypeInSpecific(file, specific_id,
-                           file.insts().Get(return_slot_pattern_id).type_id());
+  return GetTypeOfInstInSpecific(file, specific_id, return_slot_pattern_id);
 }
 
 }  // namespace Carbon::SemIR