[Ada] Fix validity checking on renamed Old attribute prefix

["Marc Poulhiès" <poulhies@adacore.com>]

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Prefix of attribute Old is captured either as an object renaming or as
an object declaration. This is now taken into account when applying
validity checks.

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

gcc/ada/

	* exp_attr.adb (Expand_N_Attribute_Reference [Attribute_Old]):
	Adapt to object declaration being rewritten into object renaming.

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diff --git a/gcc/ada/exp_attr.adb b/gcc/ada/exp_attr.adb
--- a/gcc/ada/exp_attr.adb
+++ b/gcc/ada/exp_attr.adb
@@ -5088,7 +5088,17 @@ package body Exp_Attr is
          --  to reflect the new placement of the prefix.
 
          if Validity_Checks_On and then Validity_Check_Operands then
-            Ensure_Valid (Expression (Decl));
+
+            --  Object declaration that captures the attribute prefix might
+            --  be rewritten into object renaming declaration.
+
+            if Nkind (Decl) = N_Object_Declaration then
+               Ensure_Valid (Expression (Decl));
+            else
+               pragma Assert (Nkind (Decl) = N_Object_Renaming_Declaration
+                              and then Is_Rewrite_Substitution (Decl));
+               Ensure_Valid (Name (Decl));
+            end if;
          end if;
 
          Rewrite (N, New_Occurrence_Of (Temp, Loc));