[Ada] Fix 'Last_Bit attribute applied to zero-sized type

[Eric Botcazou]

The value of the attribute should be -1 if an object of such a type is placed 
on a storage unit boundary, but the compiler was generating Storage_Error.

Tested on x86_64-suse-linux, applied on all active branches.


2013-05-26  Eric Botcazou  <ebotcazou@adacore.com>

	* gcc-interface/trans.c (Attribute_to_gnu) <Attr_Last_Bit>: Add kludge
	to avoid generating an overflow for -1.


2013-05-26  Eric Botcazou  <ebotcazou@adacore.com>

	* gnat.dg/specs/last_bit.ads: New test.


-- 
Eric Botcazou
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