[Ada] Don't examine all discriminants when looking for the first one

[Pierre-Marie de Rodat]

A minor performance improvement; semantics is unaffected.

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

gcc/ada/

        * sem_ch3.adb (Build_Discriminant_Constraints): Exit once a
        first discriminant is found and the Discrim_Present flag is set.
diff --git a/gcc/ada/sem_ch3.adb b/gcc/ada/sem_ch3.adb
--- a/gcc/ada/sem_ch3.adb
+++ b/gcc/ada/sem_ch3.adb
@@ -10392,6 +10392,7 @@ package body Sem_Ch3 is
               (Discr_Expr (J), Check_Concurrent => True)
          then
             Discrim_Present := True;
+            exit;
          end if;
       end loop;