einfo.ads (Address_Taken): Document use for the second argument of Asm_Input and Asm_Output attributes.

	* einfo.ads (Address_Taken): Document use for the second argument of
	Asm_Input and Asm_Output attributes.
	* sem_attr.adb (Analyze_Attribute) <Attribute_Asm_Input>: If the second
	argument is an entity name, then set Address_Taken on it.
	<Attribute_Asm_Output>: Likewise.
	* gcc-interface/trans.c (lvalue_required_for_attribute_p): Handle the
	Attr_Asm_Input and Attr_Asm_Output attributes explicitly.
	(gnat_to_gnu) <N_Code_Statement>: If an operand is going to end up in
	memory and is a CONST_DECL, retrieve its corresponding VAR_DECL.

From-SVN: r175171