[Ada] Allow foreign exceptions

[Arnaud Charlet <charlet at adacore dot com>]

With this new feature, foreign exceptions can be caught in 'when others'
handlers and will correctly propagate through Ada frames (including at-end
handlers).

No pure Ada test.

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

2011-08-29  Tristan Gingold  <gingold@adacore.com>

	* a-exexpr-gcc.adb (Unwind_Exception): Remove default value, made it
	convention C.
	(GCC_Exception_Access): New type.
	(Unwind_DeleteException): New imported procedure
	(Foreign_Exception): Import it.
	(GNAT_GCC_Exception): Simply have the occurrence inside.
	(To_GCC_Exception): New function.
	(To_GNAT_GCC_Exception): New function.
	(GNAT_GCC_Exception_Cleanup): New procedure..
	(Propagate_GCC_Exception): New procedure.
	(Reraise_GCC_Exception): New procedure.
	(Setup_Current_Excep): New procedure.
	(CleanupUnwind_Handler): Change type of UW_Exception parameter.
	(Unwind_RaiseException): Ditto.
	(Unwind_ForcedUnwind): Ditto.
	(Remove): Removed.
	(Begin_Handler): Change type of parameter.
	(End_Handler): Ditto. Now delete the exception if still present.
	(Setup_Key): Removed.
	(Is_Setup_And_Not_Propagated): Removed.
	(Set_Setup_And_Not_Propagated): Ditto.
	(Clear_Setup_And_Not_Propagated): Ditto.
	(Save_Occurrence_And_Private): Ditto.
	(EID_For): Add 'not null' constraint on parameter.
	(Setup_Exception): Does nothing.
	(Propagate_Exception): Simplified.
	* exp_ch11.adb (Expand_N_Raise_Statement): In back-end exception model,
	re-raise is not expanded anymore.
	* s-except.ads (Foreign_Exception): New exception - placeholder for
	non Ada exceptions.
	* raise-gcc.c (__gnat_setup_current_excep): Declare
	(CXX_EXCEPTION_CLASS): Define (not yet used)
	(GNAT_EXCEPTION_CLASS): Define.
	(is_handled_by): Handle foreign exceptions.
	(PERSONALITY_FUNCTION): Call __gnat_setup_current_excep.

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