[Ada] Avoid spurious error in GNATprove mode on non-null access types

GNATprove directly handles non-null access checks, and requires that the
frontend does not insert explicit checks in the form of conditional
exceptions being raised. Now fixed.

There is no impact on compilation.

2019-08-21  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* checks.adb (Install_Null_Excluding_Check): Do not install
	check in GNATprove mode.

From-SVN: r274780

gcc/ada/ChangeLog

 2019-08-21  Yannick Moy  <moy@adacore.com>
 
+	* checks.adb (Install_Null_Excluding_Check): Do not install
+	check in GNATprove mode.
+
+2019-08-21  Yannick Moy  <moy@adacore.com>
+
 	* sem_spark.adb (Process_Path): Do nothing on address of
 	subprogram.
 

gcc/ada/checks.adb

@@ -7964,6 +7964,12 @@ package body Checks is
          return;
       end if;
 
+      --  In GNATprove mode, we do not apply the check
+
+      if GNATprove_Mode then
+         return;
+      end if;
+
       --  Otherwise install access check
 
       Insert_Action (N,