[Ada] Plug loophole in Possible_Type_For_Conditional_Expression

[Pierre-Marie de Rodat via Gcc-patches]

We need to check that the designated subprograms of access-to-subprogram
types are subtype conformant before registering a common interpretation.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

        * sem_ch4.adb (Possible_Type_For_Conditional_Expression): Add
        test for subtype conformance in the cases of
        access-to-subprogram types.
diff --git a/gcc/ada/sem_ch4.adb b/gcc/ada/sem_ch4.adb
--- a/gcc/ada/sem_ch4.adb
+++ b/gcc/ada/sem_ch4.adb
@@ -7904,11 +7904,15 @@ package body Sem_Ch4 is
 
       elsif Is_Access_Subprogram_Attribute (T1)
         and then Is_Access_Subprogram_Attribute (T2)
+        and then
+          Subtype_Conformant (Designated_Type (T1), Designated_Type (T2))
       then
          return T1;
 
       elsif Is_Access_Protected_Subprogram_Attribute (T1)
         and then Is_Access_Protected_Subprogram_Attribute (T2)
+        and then
+          Subtype_Conformant (Designated_Type (T1), Designated_Type (T2))
       then
          return T1;