[Ada] Fix handling of extended return

[Arnaud Charlet <charlet at adacore dot com>]

Tested on i686-linux, committed on trunk

Fix handling of extended return, and in particular:
The compiler blows up when processing a return from a primitive overriding
function with a limited result type in the case where the function does not
have a separate declaration. The needed implicit BIP_Alloc_Form formal was
not created for such a function because the test for Has_Controlling_Formal
in Create_Extra_Formals was returning false (due to not being set yet since
New_Overloaded_Subprogram is called too late in the case of a body with no
separate declaration). This is fixed by treating all functions with limited
tagged results as equivalent to the unconstrained result subtype case, rather
than restricting this treatment to functions with controlling results. The
additional overhead is minor and most functions with tagged results are
primitive in any case, plus this is necessary for proper treatment of calls
through an access-to-subprogram value since such calls may be directed to
both primitive and nonprimitive subprograms. A fix was also made to rewrite
the target of the initialization assignment sometimes created for the first
branch of the if statement generated for the BIP_Alloc_Form case, which needed
to be assigning to a dereference of the BIP_Object_Access formal rather than
the result object renaming (the problem being that the renaming declaration
appears later than the assignment, resulting in a premature reference).

gnat.dg/bip_prim_func.adb must compile quietly.

2007-06-06  Ed Schonberg  <schonberg@adacore.com>
	    Gary Dismukes  <dismukes@adacore.com>
	    Javier Miranda  <miranda@adacore.com>

	* exp_ch5.adb (Expand_N_Assignment_Statement): For the assignment of a
	controlled type, use Make_Handler_For_Ctrl_Operation to construct the
	required exception handler.
	(Expand_Simple_Function_Return, Expand_N_Return_Statement): Handle
	properly the case of a function whose return type is a limited
	class-wide interface type. Modify the code of the accessibility
	check to handle class-wide interface objects. In this case we need to
	displace "this" to reference the primary dispatch table to get access
	to the TSD of the object (to evaluate its accessibility level).
	(Expand_N_Extended_Return_Statement): Test for a tagged result type
	rather than a controlling result as one of the conditions for
	generating tests of the implicit BIP_Alloc_Form formal. The
	initialization assignment is also handled according to whether the
	result is tagged instead of controlling.
	In the case where the init assignment is inserted in the "then" part of
	the allocation conditional, rewrite the target to be a dereference of
	the implicit BIP_Object_Access formal.
	If the returned value is unconstrained and created on the secondary
	stack, mark the enclosing block and function so that the secondary
	stack is not reclaimed on return.
	Treat returns from functions with controlling results similarly to
	returns from functions with unconstrained result subtypes.
	If the object returned is unconstrained, and an allocator must be
	created for it, analyze the allocator once the block for the extended
	return is installed, to ensure that finalizable components
	of the expression use the proper finalization list. Guard the call to
	Move_Final_List with a check that there is something to finalize.
	(Make_Tag_Ctrl_Assignment): Use "old" handling
	of controlled type assignment for virtual machines, since new code uses
	unsupported features (such as direct access to bytes in memory).

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