Fix assembler name collision

Fix assembler name collision

Gigi uses a dummy global variable to register global types for debug
info purposes and its name can now collide with user variables.

gcc/ada/ChangeLog:
	* gcc-interface/trans.c (lvalue_for_aggregate_p): Also return true
	for return statements.
	* gcc-interface/utils.c (gnat_write_global_declarations): Use the
	maximum index for the dummy object to avoid a name collision.