[Ada] Relax null exclusion mismatch check in Relaxed_RM_Semantics mode

[Pierre-Marie de Rodat]

This is useful for e.g. CodePeer usage.

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

gcc/ada/

        * sem_ch6.adb (Null_Exclusions_Match): Relax null exclusion
        mismatch check when Relaxed_RM_Semantics is set.
diff --git a/gcc/ada/sem_ch6.adb b/gcc/ada/sem_ch6.adb
--- a/gcc/ada/sem_ch6.adb
+++ b/gcc/ada/sem_ch6.adb
@@ -6265,7 +6265,9 @@ package body Sem_Ch6 is
 
             --  Null exclusion must match
 
-            if not Null_Exclusions_Match (Old_Formal, New_Formal) then
+            if not Relaxed_RM_Semantics
+              and then not Null_Exclusions_Match (Old_Formal, New_Formal)
+            then
                Conformance_Error
                  ("\null exclusion for& does not match", New_Formal);