Add testcase for PR lto/119792

It demonstrates a serious LTO breakage for the Ada language.

gcc/testsuite/
	PR lto/119792
	* gnat.dg/lto29.adb: New test.
	* gnat.dg/lto29_pkg.ads: New helper.

gcc/testsuite/gnat.dg/lto29.adb

@@ -0,0 +1,9 @@
+-- { dg-do run }
+-- { dg-options "-O -flto" { target lto } }
+
+with Lto29_Pkg;
+
+procedure Lto29 is
+begin
+  null;
+end;

gcc/testsuite/gnat.dg/lto29_pkg.ads

@@ -0,0 +1,15 @@
+with Ada.Strings.Bounded;
+
+package Lto29_Pkg is
+
+  package M is new Ada.Strings.Bounded.Generic_Bounded_Length (10);
+
+  type T is new M.Bounded_String;
+
+  Null_T : constant T;
+
+private
+
+  Null_T : constant T := To_Bounded_String ("");
+
+end Lto29_Pkg;