ada: Implement From_Address attribute

ada: Implement From_Address attribute

Add support for the GNAT-defined From_Address attribute.

gcc/ada/ChangeLog:

	* doc/gnat_rm/implementation_defined_attributes.rst: Document the
	From_Address attribute.
	* exp_attr.adb (Expand_N_Attribute_Reference): Implement expansion
	of a From_Address attribute reference.
	* sem_attr.adb (Analyze_Attribute): Implement analysis of a
	From_Address attribute reference.
	(Eval_Attribute): A From_Address attribute reference cannot be
	statically evaluated.
	* snames.ads-tmpl: Declare Name_From_Address and Attribute_From_Address.
	* gnat_rm.texi: Regenerate.
	* gnat_ugn.texi: Regenerate.