* sem_ch12.adb (Analyze_Subprogram_Instantiation): Do not suppress
style checks, because the subprogram instance itself may contain
violations of syle rules.
* style.adb (Missing_Overriding): Check for missing overriding
indicator on a subprogram instance.
* sem_ch6.adb (Last_Implicit_Declaration): New routine.
(Process_PPCs): Insert the body of _postconditions after the
last internally generated declaration. This ensures that actual
subtypes created for formal parameters are visible and properly
frozen as _postconditions may reference them.