[Ada] Reuse Has_Defaulted_Discriminants where possible
gcc/ada/
* exp_attr.adb, exp_ch9.adb, sem_ch3.adb: Reuse
Has_Defaulted_Discriminants.
* sem_ch4.adb (Analyze_Allocator): Reuse
Has_Defaulted_Discriminants (after reordering conjuncts); remove
redundant IF statement, whose condition is implied by
Has_Defaulted_Discriminants.
* sem_util.adb (Has_Defaulted_Discriminants): Has_Discriminants
implies that the First_Discriminant is present.
(Is_Fully_Initialized_Type): Reuse Has_Defaulted_Discriminants.