[Ada] Disable certain checks in predefined units

[Pierre-Marie de Rodat <derodat@adacore.com>]

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This patch fixes a failure of ACATS test cxaib08, which was getting
errors in instances of Bounded_Hashed_Maps about nonpreelaborable
constructs.

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

gcc/ada/

	* sem_cat.adb (Check_Non_Static_Default_Expr): Allow nonstatic
	expression in predefined unit with pragma Preelaborate.

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diff --git a/gcc/ada/sem_cat.adb b/gcc/ada/sem_cat.adb
--- a/gcc/ada/sem_cat.adb
+++ b/gcc/ada/sem_cat.adb
@@ -356,6 +356,14 @@ package body Sem_Cat is
          if Present (Expression (Component_Decl))
            and then Nkind (Expression (Component_Decl)) /= N_Null
            and then not Is_OK_Static_Expression (Expression (Component_Decl))
+
+           --  If we're in a predefined unit, we can put whatever we like in a
+           --  preelaborated package, and in fact in some cases it's necessary
+           --  to bend the rules. Ada.Containers.Bounded_Hashed_Maps contains
+           --  some code that would not be considered preelaborable in user
+           --  code, for example.
+
+           and then not In_Predefined_Unit (Component_Decl)
          then
             Error_Msg_Sloc := Sloc (Component_Decl);
             Error_Msg_F